Web3Signer support for VC (#2522)

[EIP-3030]: https://eips.ethereum.org/EIPS/eip-3030
[Web3Signer]: https://consensys.github.io/web3signer/web3signer-eth2.html

## Issue Addressed

Resolves #2498

## Proposed Changes

Allows the VC to call out to a [Web3Signer] remote signer to obtain signatures.


## Additional Info

### Making Signing Functions `async`

To allow remote signing, I needed to make all the signing functions `async`. This caused a bit of noise where I had to convert iterators into `for` loops.

In `duties_service.rs` there was a particularly tricky case where we couldn't hold a write-lock across an `await`, so I had to first take a read-lock, then grab a write-lock.

### Move Signing from Core Executor

Whilst implementing this feature, I noticed that we signing was happening on the core tokio executor. I suspect this was causing the executor to temporarily lock and occasionally trigger some HTTP timeouts (and potentially SQL pool timeouts, but I can't verify this). Since moving all signing into blocking tokio tasks, I noticed a distinct drop in the "atttestations_http_get" metric on a Prater node:

![http_get_times](https://user-images.githubusercontent.com/6660660/132143737-82fd3836-2e7e-445b-a143-cb347783baad.png)

I think this graph indicates that freeing the core executor allows the VC to operate more smoothly.

### Refactor TaskExecutor

I noticed that the `TaskExecutor::spawn_blocking_handle` function would fail to spawn tasks if it were unable to obtain handles to some metrics (this can happen if the same metric is defined twice). It seemed that a more sensible approach would be to keep spawning tasks, but without metrics. To that end, I refactored the function so that it would still function without metrics. There are no other changes made.

## TODO

- [x] Restructure to support multiple signing methods.
- [x] Add calls to remote signer from VC.
- [x] Documentation
- [x] Test all endpoints
- [x] Test HTTPS certificate
- [x] Allow adding remote signer validators via the API
- [x] Add Altair support via [21.8.1-rc1](https://github.com/ConsenSys/web3signer/releases/tag/21.8.1-rc1)
- [x] Create issue to start using latest version of web3signer. (See #2570)

## Notes

- ~~Web3Signer doesn't yet support the Altair fork for Prater. See https://github.com/ConsenSys/web3signer/issues/423.~~
- ~~There is not yet a release of Web3Signer which supports Altair blocks. See https://github.com/ConsenSys/web3signer/issues/391.~~

testing/web3signer_tests/build.rs

@@ -0,0 +1,100 @@
+//! This build script downloads the latest Web3Signer release and places it in the `OUT_DIR` so it
+//! can be used for integration testing.
+
+use reqwest::Client;
+use serde_json::Value;
+use std::env;
+use std::fs;
+use std::path::PathBuf;
+use zip::ZipArchive;
+
+/// Use `None` to download the latest Github release.
+/// Use `Some("21.8.1")` to download a specific version.
+const FIXED_VERSION_STRING: Option<&str> = None;
+
+#[tokio::main]
+async fn main() {
+    let out_dir = env::var("OUT_DIR").unwrap();
+    download_binary(out_dir.into()).await;
+}
+
+pub async fn download_binary(dest_dir: PathBuf) {
+    let version_file = dest_dir.join("version");
+
+    let client = Client::builder()
+        // Github gives a 403 without a user agent.
+        .user_agent("web3signer_tests")
+        .build()
+        .unwrap();
+
+    let version = if let Some(version) = FIXED_VERSION_STRING {
+        version.to_string()
+    } else {
+        // Get the latest release of the web3 signer repo.
+        let latest_response: Value = client
+            .get("https://api.github.com/repos/ConsenSys/web3signer/releases/latest")
+            .send()
+            .await
+            .unwrap()
+            .error_for_status()
+            .unwrap()
+            .json()
+            .await
+            .unwrap();
+        latest_response
+            .get("tag_name")
+            .unwrap()
+            .as_str()
+            .unwrap()
+            .to_string()
+    };
+
+    if version_file.exists() && fs::read(&version_file).unwrap() == version.as_bytes() {
+        // The latest version is already downloaded, do nothing.
+        return;
+    } else {
+        // Ignore the result since we don't care if the version file already exists.
+        let _ = fs::remove_file(&version_file);
+    }
+
+    // Download the latest release zip.
+    let zip_url = format!("https://artifacts.consensys.net/public/web3signer/raw/names/web3signer.zip/versions/{}/web3signer-{}.zip", version, version);
+    let zip_response = client
+        .get(zip_url)
+        .send()
+        .await
+        .unwrap()
+        .error_for_status()
+        .unwrap()
+        .bytes()
+        .await
+        .unwrap();
+
+    // Write the zip to a file.
+    let zip_path = dest_dir.join(format!("{}.zip", version));
+    fs::write(&zip_path, zip_response).unwrap();
+    // Unzip the zip.
+    let mut zip_file = fs::File::open(&zip_path).unwrap();
+    ZipArchive::new(&mut zip_file)
+        .unwrap()
+        .extract(&dest_dir)
+        .unwrap();
+
+    // Rename the web3signer directory so it doesn't include the version string. This ensures the
+    // path to the binary is predictable.
+    let web3signer_dir = dest_dir.join("web3signer");
+    if web3signer_dir.exists() {
+        fs::remove_dir_all(&web3signer_dir).unwrap();
+    }
+    fs::rename(
+        dest_dir.join(format!("web3signer-{}", version)),
+        web3signer_dir,
+    )
+    .unwrap();
+
+    // Delete zip and unzipped dir.
+    fs::remove_file(&zip_path).unwrap();
+
+    // Update the version file to avoid duplicate downloads.
+    fs::write(&version_file, version).unwrap();
+}