[relay] Return raw bundle bytes from relay

[ian-shim]

Why are these changes needed?

Instead of returning structured array of frames, return raw serialized bytearrays from relay.
It delegates responsibility to deserialize byte arrays to the caller.

Checks

• 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.
• I've checked the new test coverage and the coverage percentage didn't drop.
• Testing Strategy
  • Unit tests
  • Integration tests
  • This PR is not tested :(