Allow to provide client certificate

### Feature Request

Gitlab supports [2-way SSL authentication](https://docs.gitlab.com/omnibus/settings/nginx.html#enable-2-way-ssl-client-authentication), in that case a client must provide a certificate with any HTTP request towards Gitlab server.
Today we can not use Jenkins-to-Gitlab authentication since the plugin does not allow us to provide such certificate, then Gitlab responds with Bad Request.

It would be great to be able to specify a client certificate to use in the plugin configuration.