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();

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

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();

validator_client/src/http_api/tests/keystores.rs

@@ -40,6 +40,8 @@ fn web3signer_validator_with_pubkey(pubkey: PublicKey) -> Web3SignerValidatorReq
         url: web3_signer_url(),
         root_certificate_path: None,
         request_timeout_ms: None,
+        client_identity_path: None,
+        client_identity_password: None,
     }
 }