Support GitHub copilot as a provider

[lurebat]

I want to try out the harness, but I only have GitHub copilot.

Both maki and oh-my-pi support it.

[yogthos]

That sounds reasonable, I'll take a look at adding that in the next few days.

[lurebat]

@yogthos there's no contributors docs, can i contribute it? Any instructions or best practices?

[yogthos]

Yeah, I haven't had a chance to get around to contributor docs yet, but PRs are welcome. In terms of best practices, ideally if you can make features self contained, and following existing code structure and style. There's rustfmt in the CI build, so that will catch any style related issues.