Add repository content management functions

[MariusStorhaug]

https://docs.github.com/en/rest/repos/contents?apiVersion=2022-11-28

[MariusStorhaug]

Concrete consumer for this: MSXOrg/Custo (PR #1) currently distributes managed files by cloning every target repo on each run, which does not scale across a fleet. Content-management cmdlets would let it sync a single file via the API instead.

Specific needs for that use case:

• Get-GitHubRepositoryContent — GET /repos/{owner}/{repo}/contents/{path} (return content + sha).
• Set-GitHubRepositoryContent — PUT /repos/{owner}/{repo}/contents/{path} (create/update; requires the existing blob sha on update, and supports a target branch).
• Remove-GitHubRepositoryContent — DELETE /repos/{owner}/{repo}/contents/{path} (needed for convergence when a file is unsubscribed/removed).

Idempotency matters: callers want to compare the desired bytes against the current blob and skip the write when unchanged, so returning the current sha/content from Get is important.