feat: Add client support for response metadata

[regiskuckaertz]

@thesamet even though we can now send trailers, there is no way to consume them in the client 😓

I think the generated code should offer support for it but I would like your opinion first. Since it would change the result type, we can either:

• add *WithMetadata methods on the client impl, or
• generate an "advanced" client impl with the same method names that yields the metadata

[thesamet]

Hi @regiskuckaertz , let's create a seperate advanced client and use same-name methods. I can be convinced otherwise, but my rational for the choice is to keep interface for 99% of the users cleaner. Ideally, the simple client implementation uses the advanced clients internally through an instance it holds, so users should be to call client.withMetadata.myMethod(...`).

Once this works, please add accessor methods too that follow the same naming.

I haven't looked too closely, but I think that the advanced client should have all its methods return streams, even unary calls can return both initial and trailing metadata.

[regiskuckaertz]

@thesamet I was of the same opinion

[regiskuckaertz]

@thesamet I'll add some tests, but so far so good

[regiskuckaertz]

@thesamet check it out in your own time

[regiskuckaertz]

@thesamet ping

[thesamet]

Sorry for the delay. I'm out of town without desktop access until mid October. Overall looks good, though I'd like to take a closer look at the generated code. Can you prepare a PR for master that will be used for review? We will use this as the backport PR