TLS handling on Client API

[ignasi35]

When merging #1319 we may be able to use the grpc-java (netty client) API improvements to handle TLS certificates (in both trustmanagers and keymanagers) on the client API.

api: TlsChannelCredentials and TlsServerCredentials now support client certificates
and custom KeyManagers/TrustManagers. grpc-netty fully supports these options. grpc-okhttp
does not support keyfile-based configuration; you’d need to use a KeyManager. Most users
of Netty’s SslContext and GrpcSslContexts should be able to migrate and are encouraged to
do so, because this API does not have a Netty dependency and so is planned to become stable.

I'm not entirely sure what's the latest implementation but I think we had some coupling or class casting which we may be able to get rid of.