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.

Cargo.lock

@@ -489,6 +489,7 @@ dependencies = [
  "futures",
  "genesis",
  "hex",
+ "http_api",
  "hyper",
  "lighthouse_version",
  "monitoring_api",
@@ -6208,6 +6209,17 @@ dependencies = [
  "tokio",
 ]
 
+[[package]]
+name = "tokio-rustls"
+version = "0.22.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "bc6844de72e57df1980054b38be3a9f4702aba4858be64dd700181a8a6d0e1b6"
+dependencies = [
+ "rustls",
+ "tokio",
+ "webpki",
+]
+
 [[package]]
 name = "tokio-stream"
 version = "0.1.7"
@@ -6799,7 +6811,7 @@ dependencies = [
 [[package]]
 name = "warp"
 version = "0.3.0"
-source = "git+https://github.com/paulhauner/warp?branch=cors-wildcard#1f7daf462e6286fe5fd1743f7b788227efd3fa5c"
+source = "git+https://github.com/macladson/warp?rev=dfa259e#dfa259e19b7490e6bc4bf247e8b76f671d29a0eb"
 dependencies = [
  "bytes 1.1.0",
  "futures",
@@ -6817,6 +6829,7 @@ dependencies = [
  "serde_json",
  "serde_urlencoded",
  "tokio",
+ "tokio-rustls",
  "tokio-stream",
  "tokio-tungstenite",
  "tokio-util",