Support multiple notifications

[l3ender]

Overview

This PR adds support for sending multiple Github status notifications for a single build.

This can be useful if using multiple jobs where one is based on pull requests and the other is based on branches NOT included in pull requests. If the latter/branch build uses a status notification which is configured as a status check in Github, that status will not be reported to Github when a branch is part of a pull request. Assuming the pull request job performs at least the same functions as the branch job, it is helpful to have the pull request job configure the same status as the branch job in order to ensure the necessary statuses are reported to Github.

Testing done

After making changes:

1. Ran project locally using mvn hpi:run.
2. Installed Pipeline plugin in Jenkins.
3. Created multibranch pipeline job referencing test Github repo.
4. Added this plugin's behavior and triggered build to observe statuses being reported to Github.

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] Link to relevant issues in GitHub or Jira
- [x] Link to relevant pull requests, esp. upstream and downstream changes
- [x] Ensure you have provided tests - that demonstrates feature works or fixes the issue