GitLab API Client support for client certificates

This change adds support for client certificates when using the GitLab API client.

A new option, WithClientCert, allows a PEM encoded x509 certificate-key pair to be provided to the HttpClient. The test server was updated to require and verify client certs to demonstrate the client was properly configured.

Test certs were also created. The certs can be regenerated using the command go generate https_client.go inside the client directory.

Part of gitaly#3160 (closed)