Allow overriding the gRPC connection address #260

Allow a gRPC URI to connect to an explicit address while retaining the URI hostname for TLS verification and the HTTP/2 authority.

This is useful when testing or accessing an API through a private network without changing system DNS. For example, the client should be able to connect to [fd00:1:0:fffc::110]:443 while using dispatch.eu-1.staging.api.frequenz.com for TLS SNI, certificate verification, and :authority.

The implementation could expose this through ChannelOptions or a URI query parameter. For secure channels it should configure both grpc.ssl_target_name_override and grpc.default_authority; certificate verification must remain enabled.

Please include tests covering IPv4 and IPv6 connection addresses, TLS hostname validation, and malformed overrides.