Add integration tests with `bitcoinj`

I think eventually we'll want to write a bunch of integration tests towards `bitcoinj` and run them in CI.

While I don't expect any surprises, it might be beneficial to check especially de/serialization of all types this way.

Start by locating the existing integration-test and CI entry points, then review how bitcoinj can exercise the Rust interface. Define the serialization and deserialization types to cover before implementing the test suite. Done means the bitcoinj integration tests run in CI and cover the agreed types without unexpected mismatches.

Written by the indexing model from the issue text.