[CheckRuns] Add method for listing checkruns for given ref

[avano]

Description

• Add a method for GHCommit and GHRepository for listing the check-runs for given sha
• Add tests for getting check-run for given commit and for getting the last status of given commit
• Added missing enum for GitHub registry_package event

Before submitting a PR:

We love getting PRs, but we hate asking people for the same basic changes every time.

• Push your changes to a branch other than master. Create your PR from that branch.
• Add JavaDocs and other comments
• Write tests that run and pass in CI. See CONTRIBUTING.md for details on how to capture snapshot data.
• Run mvn -D enable-ci clean install site locally. This may reformat your code, commit those changes. If this command doesn't succeed, your change will not pass CI.

[avano]

@bitwiseman I incorporated the suggested change manually, as after accepting it the build complained about it being incorrectly formatted 😀

[bitwiseman]

This is good, but it needs to use the paging framework. This is in the api documentation, but is still non-obvious. Sorry.

[avano]

also added missing registry_package event that caused problems when fetching check runs from github-api repository

[bitwiseman]

Looks good to me.