Temporarily expose gRPC client connection for cloud API use

[cretz]

Describe the solution you'd like

The in-development cloud API wants to reuse the gRPC connection from the Go client to instantiate the gRPC API. This API is not yet stable and may move to a different location at some point, so this is just a temporary need at this time.

I think a GetGRPCConnection(internal.Client) *grpc.ClientConn should be added to https://pkg.go.dev/go.temporal.io/sdk/internalbindings with a warning that it is experimental and may be removed in the future.