Add TLS capability to the beacon node HTTP API (#2668)

Currently, the beacon node has no ability to serve the HTTP API over TLS.
Adding this functionality would be helpful for certain use cases, such as when you need a validator client to connect to a backup beacon node which is outside your local network, and the use of an SSH tunnel or reverse proxy would be inappropriate.

## Proposed Changes

- Add three new CLI flags to the beacon node
  - `--http-enable-tls`: enables TLS
  - `--http-tls-cert`: to specify the path to the certificate file
  - `--http-tls-key`: to specify the path to the key file
- Update the HTTP API to optionally use `warp`'s [`TlsServer`](https://docs.rs/warp/0.3.1/warp/struct.TlsServer.html) depending on the presence of the `--http-enable-tls` flag
- Update tests and docs
- Use a custom branch for `warp` to ensure proper error handling

## Additional Info

Serving the API over TLS should currently be considered experimental. The reason for this is that it uses code from an [unmerged PR](https://github.com/seanmonstar/warp/pull/717). This commit provides the `try_bind_with_graceful_shutdown` method to `warp`, which is helpful for controlling error flow when the TLS configuration is invalid (cert/key files don't exist, incorrect permissions, etc). 
I've implemented the same code in my [branch here](https://github.com/macladson/warp/tree/tls).

Once the code has been reviewed and merged upstream into `warp`, we can remove the dependency on my branch and the feature can be considered more stable.

Currently, the private key file must not be password-protected in order to be read into Lighthouse.

lighthouse/tests/beacon_node.rs

@@ -573,6 +573,30 @@ fn http_allow_origin_all_flag() {
         .run()
         .with_config(|config| assert_eq!(config.http_api.allow_origin, Some("*".to_string())));
 }
+#[test]
+fn http_tls_flags() {
+    let dir = TempDir::new().expect("Unable to create temporary directory");
+    CommandLineTest::new()
+        .flag("http-enable-tls", None)
+        .flag(
+            "http-tls-cert",
+            dir.path().join("certificate.crt").as_os_str().to_str(),
+        )
+        .flag(
+            "http-tls-key",
+            dir.path().join("private.key").as_os_str().to_str(),
+        )
+        .run()
+        .with_config(|config| {
+            let tls_config = config
+                .http_api
+                .tls_config
+                .as_ref()
+                .expect("tls_config was empty.");
+            assert_eq!(tls_config.cert, dir.path().join("certificate.crt"));
+            assert_eq!(tls_config.key, dir.path().join("private.key"));
+        });
+}
 
 // Tests for Metrics flags.
 #[test]