feat: Add Squash merge commit message/title

[fatbasstard]

This PR adds the configuration of the squash commit message/title. Will be specified when allow_squash_merge is true.

Tested this out, and the commit title/message have a default value, so configured this explicitly for simplicity:

  ~ resource "github_repository" "default" {
      ~ allow_squash_merge          = false -> true
      ~ squash_merge_commit_message = "COMMIT_MESSAGES" -> "BLANK"
      ~ squash_merge_commit_title   = "COMMIT_OR_PR_TITLE" -> "PR_TITLE"
        # (33 unchanged attributes hidden)
    }

Links:

• https://github.blog/changelog/2022-08-23-new-options-for-controlling-the-default-commit-message-when-merging-a-pull-request/
• https://registry.terraform.io/providers/integrations/github/latest/docs/resources/repository

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