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/tests.rs

@@ -457,6 +457,8 @@ impl ApiTester {
                     url: format!("http://signer_{}.com/", i),
                     root_certificate_path: None,
                     request_timeout_ms: None,
+                    client_identity_path: None,
+                    client_identity_password: None,
                 }
             })
             .collect();