feat: barretenberg PR migrate tool

[ludamad]

Description

Originally this was going to be a short video, then I hit edge cases. I went for a script. Hopefully I can do a video in a bit explaining what is needed to know to understand this, but after this it should be a one liner:

./scripts/migrate_barretenberg_branch.sh <barretenberg-branch> "<commit-message>"

There is a fair bit of error handling and recovery advice as this is a cross-repo stitching and thus edge-case prone.
I hit one big gotcha here that is worked around with any such operation briefly doing a git rm <folder>, and git rm updating .gitmodules surprisingly. This gets rid of this diff, which is 'expected' when git submodules interact with git subrepo.
Also it tosses away the .gitrepo changes, as those we only want if pulling from barretenberg master, not PRs (which will go in as aztec commits).

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.

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[codygunton]

I used this to create #1195--worked like a charm. Thanks for making this easy.

[spalladino]

LGTM!

This is an edge-case gotcha using git submodules along with git-subrepo.

Should we consider moving completely away from submodules, once we get comfortable with subrepo?