Feature Request: Support looking up GH App installation ID through the GH owner

[matheuscscp]

It's easier for Flux users if they don't have to look up the GH App installation ID. There aren't good GH docs on this, the best you get googling for it is the following:

https://www.google.com/search?q=github+app+installation+id&oq=github+app+installation+id&gs_lcrp=EgZjaHJvbWUyCQgAEEUYORiABDIICAEQABgWGB4yCAgCEAAYFhgeMggIAxAAGBYYHjIICAQQABgWGB4yBggFEEUYPDIGCAYQRRhAMgYIBxBFGEDSAQgzODA3ajBqN6gCALACAA&sourceid=chrome&ie=UTF-8

https://stackoverflow.com/questions/74462420/where-can-we-find-github-apps-installation-id

So we can introduce a new Secret key called githubAppInstallationOwner and, if that's set instead of githubAppInstallationID, we look up the installation ID using the owner.