feat: ProtoGalaxy Combiner

[codygunton]

Adds the computation of the combiner polynomial $G$.

Checklist:

Remove the checklist to signal you've completed it. Enable auto-merge if the PR is ready to merge.

• If the pull request requires a cryptography review (e.g. cryptographic algorithm implementations) I have added the 'crypto' tag.
• I have reviewed my diff in github, line by line and removed unexpected formatting changes, testing logs, or commented-out code.
• Every change is related to the PR description.
• I have linked this pull request to relevant issues (if any exist).

[codygunton]

Subsumed by #2436, which does the big ugly rebase over the move of barretenberg to outside of the circuits library.