gRPC: GHC 9.12 support

[carbolymer]

Changelog

- description: |
    Add GHC 9.12 support for cardano-rpc
# uncomment types applicable to the change:
  type:
  # - feature        # introduces a new feature
  # - breaking       # the API has changed in a breaking way
  # - compatible     # the API has changed but is non-breaking
  # - optimisation   # measurable performance improvements
  # - refactoring    # QoL changes
  # - bugfix         # fixes a defect
  # - test           # fixes/modifies tests
   - maintenance    # not directly related to the code
  # - release        # related to a new release preparation
  # - documentation  # change in code docs, haddocks...
# uncomment at least one main project this PR is associated with
  projects:
   - cardano-api
  # - cardano-api-gen
   - cardano-rpc
  # - cardano-wasm

Context

This PR reenables cardano-rpc and adds GHC 9.12 support. This requires a proto-lens SRP with changes not merged yet.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated. See Running tests for more details
• Self-reviewed the diff

[palas]

Nice! Looks good and CI passes. Just a couple of minor comments