Add GithubSenderCause to PR and push GHEventSubscribers

[mk7run]

Description

This allows us to infer the person responsible for opening a PR or
updating a branch which triggers a Jenkins build. We extract this info
inside a Jenkins pipeline like so:

def causes =
currentBuild.getBuildCauses('org.jenkinsci.plugins.github_branch_source.GitHubSenderCause')
if (!causes.isEmpty()) {
  return causes[0].login
}

Example pipeline utilizing above code:

To run updated unit tests:

mvn test -Dtest=EventsTest.java

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Automated tests have been added to exercise the changes
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verify that the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given

Documentation changes

• Link to jenkins.io PR, or an explanation for why no doc changes are needed

Users/aliases to notify

[wlfshmn]

Is there still interest in this PR? I'm using this in my instance to gain visibility and auditability into who does what in our builds, and would very much like to see this updated and merged upstream.