Provide an 'architecture request' tag for apio issues

[zapta]

We have here an issue tag 'board request'. Please create also an 'architecture request' and use it for existing requests for architectures apio doesn't support it.

For multiple requests for the same architecture, even if the boards are different, I think it's reasonable to mark them as dupes of each other, having only one open issue per new architecture.

As for handling boards requests for existing architecture, I suggest to ask the user to try them with custom boards.json in their project and then submit a PR once they pass, hopefully with an example or two for the apio-examples repo.

@Obijuan, @cavearr, please take a look.