feat: add authority header setup capability

[nicholasxuu]

No description provided.

[notmedia]

Hi, thank you for opening this PR! I have some points we need to discuss:

1. Could you elaborate about your case and infrastructure, why you need this option? I want to reproduce it and make some tests.
2. If the grpc.default_authority is set you are also setting the same value to the grpc.ssl_target_name_override option. I think this can break some scenarios where the values might be different. But need investigate this, for example - Negotiating TLS when using authority for Envoy routing grpc/grpc-node#655
3. I moved grpc-client from this project to separate library - https://github.com/getezy/grpc-client where I will add this option too after investigating.