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.

testing/web3signer_tests/tls/lighthouse/cert.pem

@@ -0,0 +1,32 @@
+-----BEGIN CERTIFICATE-----
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+10XBwkOm1CgvZxHaj4P0NJf+wih+K8Z5Dg1+90nnJ4mxGFFIW8m7Cfn1tPFmEPo=
+-----END CERTIFICATE-----

testing/web3signer_tests/tls/lighthouse/config

@@ -0,0 +1,19 @@
+[req]
+default_bits = 4096
+default_md = sha256
+distinguished_name = req_distinguished_name
+x509_extensions = v3_req
+prompt = no
+[req_distinguished_name]
+C = US
+ST = VA
+L = SomeCity
+O = MyCompany
+OU = MyDivision
+CN = lighthouse
+[v3_req]
+keyUsage = keyEncipherment, dataEncipherment
+extendedKeyUsage = serverAuth
+subjectAltName = @alt_names
+[alt_names]
+IP.1 = 127.0.0.1

testing/web3signer_tests/tls/lighthouse/key.key

@@ -0,0 +1,52 @@
+-----BEGIN PRIVATE KEY-----
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+-----END PRIVATE KEY-----

testing/web3signer_tests/tls/lighthouse/password.txt

@@ -0,0 +1 @@
+bark

testing/web3signer_tests/tls/lighthouse/web3signer.pem

@@ -0,0 +1,32 @@
+-----BEGIN CERTIFICATE-----
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+-----END CERTIFICATE-----