GitHub App authentication for Git Trigger Source

[bkanuka]

We can currently use git as a template trigger source https://argoproj.github.io/argo-events/tutorials/03-trigger-sources/#git
but this requires username+password or ssh key. Cloning a repo with GitHub App credentials requires generating a JWT access token and using that as the password.

I already have a GitHub app set up for using as an EventSource, so it would be ideal if these same app credentials could be used for accessing the repo content as a Trigger Source.

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