[notifiers/github] Add display name annotation.

[wlynch]

Changes

Adds an optional annotation to control what the statuses display name in
GitHub. If not specified, this defaults to the Task's name.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes docs (if user facing)
• Commit messages follow commit message best practices
• Commit messages includes a project tag in the subject - e.g. [OCI], [hub], [results], [task-loops]

See the contribution guide
for more details.

Fixes #779

[tekton-robot]

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This pull-request has been approved by: sbwsg

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• notifiers/github-app/OWNERS [sbwsg]

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[dibyom]

/lgtm