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.

validator_client/Cargo.toml

@@ -43,7 +43,7 @@ eth2_keystore = { path = "../crypto/eth2_keystore" }
 account_utils = { path = "../common/account_utils" }
 lighthouse_version = { path = "../common/lighthouse_version" }
 warp_utils = { path = "../common/warp_utils" }
-warp = { git = "https://github.com/paulhauner/warp ", branch = "cors-wildcard" }
+warp = { git = "https://github.com/macladson/warp", rev ="dfa259e" }
 hyper = "0.14.4"
 eth2_serde_utils = "0.1.0"
 libsecp256k1 = "0.6.0"