Add support for configuring default squash and merge commit titles and messages

[soerenmartius]

[soerenmartius]

@mariux is this good to go?

[soerenmartius]

@mariux the tests are failing after applying your provided suggestions ... not a priority though https://github.com/mineiros-io/terraform-github-repository/actions/runs/3394710192/jobs/5643646371

[anthr76]

Is this ready to be merged?

[gaima8]

Is this PR still actually a draft, or once conflicts are resolved, is it ready for approval and merging? Githubs default squash strategy grinds my gears.

Thanks

[bodgit]

Would be good to get this over the line. I can't even manually tweak the repository settings as the current release of this module resets the squash settings back to the defaults.

[marcjay]

I think this PR might be ready to review?

[marko-fabry]

Hello all! What is the ETA on merging this one? We forked it for the time being just for this PR and it seems to work as expected.