Implement github reporter

[La0]

Part of #2680

We need a new implementation of base reporter for Github, similar to PhabricatorReporter

The GithubReporter class only needs to:

• authenticate on Github using a configuration (from Taskcluster secret)
• implement the publish method that is called by the workflow

You'll need to add the new reporter into get_reporters.

The publication method should be really similar to Phabricator (at first):

1. create a pull request through API
2. create 1 review comment for each issue
3. submit the review, where the overall comment summarizes all the issue (we'll need to abstract the build_comment from phabricator comment so it produces a similar content for github)