Add GetRepository() to github pkg and RepoIsForkOf() to verify inheritance

[puerco]

What type of PR is this?

/kind feature

What this PR does / why we need it:

This PR Adds the following features to the github package:

• It adds a new function GetRepository() to the github package that returns a github.Repository from the GitHub API
• It adds implementations of this function to the real client, githubNotesRecordClient, githubNotesReplayClient and to the fake client.
• It implements a new function in the github package called RepoIsForkOf() which verifies that one repo is a fork of another using the github API.
• Adds tests to github_test.go for said functions

Which issue(s) this PR fixes:

Relates to #834

Special notes for your reviewer:

I've tried to emulate the record/replay functionality by following the patterns found in the code. But I'm not familiar with that feature, please let me know if they are implemented right.

Does this PR introduce a user-facing change?

* Add `RepoIsForkOf()` to check repository inheritance.
* Add `GetRepository()` functions in all clients to get repository data from the GitHub API.

[saschagrunert]

Nice!

/lgtm

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: puerco, saschagrunert

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• pkg/OWNERS [saschagrunert]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment