Support merging of a pull request

[tobia80]

Does the library provide support for merging a pull request?
https://docs.github.com/en/rest/pulls/pulls?apiVersion=2022-11-28#merge-a-pull-request

[fedefernandez]

@tobia80, not from what I can see in PullRequests.scala.

#818 and #811 are good examples of how to add new endpoints in case someone wants to contribute.