Add client authentication to Web3Signer validators (#3170)

## Issue Addressed

Web3Signer validators do not support client authentication. This means the `--tls-known-clients-file` option on Web3Signer can't be used with Lighthouse.

## Proposed Changes

Add two new fields to Web3Signer validators, `client_identity_path` and `client_identity_password`, which specify the path and password for a PKCS12 file containing a certificate and private key. If `client_identity_path` is present, use the certificate for SSL client authentication.

## Additional Info

I am successfully validating on Prater using client authentication with Web3Signer and client authentication.

validator_client/src/http_api/remotekeys.rs

@@ -124,6 +124,8 @@ fn import_single_remotekey<T: SlotClock + 'static, E: EthSpec>(
             url,
             root_certificate_path: None,
             request_timeout_ms: None,
+            client_identity_path: None,
+            client_identity_password: None,
         },
     };
     handle