feat: automatic branch creation for resource 'github_repository_file'

[wparr-circle]

Resolves #2101

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Before the change?

• Currently repository_file does not support automatic creation of branches. This means in order manage files via pull requests (rather than directly pushing to the branch ie. main), a branch would need created. This has impacts on terraform lifecycle post-creation ie. a pull request merging the branch and deleting it, in which the branch would attempt to be re-created causing a long-lived branch to exist in a repo.

After the change?

• When the flag autocreate_branch is set true, if the branch does not already exist then it shall be created based on autocreate_branch_source_sha or autocreate_branch_source_branch which defaults to main during create, update or destroy. Given the branch could be ephemeral; on reads, if the branch does not exist then content shall be read from autocreate_branch_source_branch as it may been merged.

Pull request checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been reviewed and added / updated if needed (for bug fixes / features)

Does this introduce a breaking change?

Please see our docs on breaking changes to help!

• Yes
• No

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[kfcampbell]

@wparr-circle there's a pretty quick go fmt issue to resolve here, then I can get this merged and released.

[wparr-circle]

@kfcampbell applied go fmt changes now. Thanks