Support GitHub annotation formatter

[grobie]

User story. As a developer, I can configure the formatter to use the GitHub annotation format, so that I can see warnings directly in my pull request when running spectral as Github action.

Is your feature request related to a problem? Developers are often missing new Spectral warnings as they rarely check the logs of the Github action run manually.

Describe the solution you'd like I'd expect to be able to use --format github-annotation or similar as CLI parameter.

Additional context Github annotations support both warnings and errors, as seen in this example:

[philsturgeon]

@grobie this would be pretty cool! Would you be prepared to make a pull request for this? I think you could copy and tweak your way to a solution, starting with CONTRIBUTING.md.