Add BatchConfirmer

[ian-shim]

Why are these changes needed?

BatchConfirmer will take the MinibatchResponses and confirm the full batch by getting & aggregating the nodes' signatures for the full batch.

Checks

• [ ] I've made sure the lint is passing in this PR.
• [ ] I've made sure the tests are passing. Note that there might be a few flaky tests, in that case, please comment that they are not relevant.
• Testing Strategy
  • [ ] Unit tests
  • [ ] Integration tests
  • [ ] This PR is not tested :(