[api] Expose used_by_txs in reference memo API endpoint

Son of Odinrequested to merge
odin/expose-used-by-txs-in-memo-endpoint into develop

Summary

  • Add the list of transaction hashes that used a reference memo to the /thorchain/memo/{asset}/{reference} API response
  • The ReferenceMemo type already stored this data internally but it was not exposed through the API

Changes

  • Add used_by_txs field to QueryReferenceMemoResponse proto message
  • Add used_by_txs field to QueryReferenceMemoByHashResponse proto message
  • Update OpenAPI spec with used_by_txs array field
  • Update queriers to populate the new field from stored data
  • Add regression test assertions to verify the field is populated

Test plan

  • Regression test memo.yaml passes with new assertions for used_by_txs
  • Build succeeds

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