feat(bb): load proving key

[Maddiaa0]

Followup to write proving key work

Please provide a paragraph or two giving a summary of the change, including relevant motivation and context.

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).

[Maddiaa0]

closing as stale - tracked in #3833