feat: bundle git-subrepo

[ludamad]

Description

This will be used by future CI. There is a bunch of efforts now that will include other repos, and there is a desire (as it's quite cheap) to not fully deprecate those repos and instead mirror them. "git subrepo" was picked as the tool of choice for being able to pull and push from a separate repo, with both being useful for noir, and particularly push being useful for bb and docs (with pull having some usefulness too until we fully move)

Checklist:

• [*] I have reviewed my diff in github, line by line.
• [*] Every change is related to the PR description.
• [*] I have linked this pull request to the issue(s) that it resolves.
• [*] There are no unexpected formatting changes, superfluous debug logs, or commented-out code.
• [*] The branch has been merged or rebased against the head of its merge target.
• [*] I'm happy for the PR to be merged at the reviewer's next convenience.