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/mod.rs

@@ -475,6 +475,8 @@ pub fn serve<T: 'static + SlotClock + Clone, E: EthSpec>(
                                     url: web3signer.url,
                                     root_certificate_path: web3signer.root_certificate_path,
                                     request_timeout_ms: web3signer.request_timeout_ms,
+                                    client_identity_path: web3signer.client_identity_path,
+                                    client_identity_password: web3signer.client_identity_password,
                                 },
                             })
                             .collect();