feat: add Ecto transaction callbacks

[pepicrft]

Related: plausible/ch#310

Summary

• implement Ecto.Adapter.Transaction for the ClickHouse adapter
• delegate transaction/3, in_transaction?/1, and rollback/2 to Ecto.Adapters.SQL
• add adapter-level tests that exercise the new transaction callbacks without requiring experimental ClickHouse transactions in CI

Why

This is the adapter half of an experimental ClickHouse transaction spike for Ecto SQL sandbox support.

Validation

• mix test test/ecto/adapters/clickhouse_test.exs

[ruslandoga]

@pepicrft 👋

Thank you for the PR! It'd take me some time to get to review it though, since I'm actually in the process of evaluating dropping db_connection ... But we still would be able to support Ecto sandbox and these transactions, I think. Ecto sandbox owns a connection because that's Postgres' transactions connection-local, but in ClickHouse we "just" need to pass session_id in the query string, so it might be even easier. We can do that with any HTTP connection from the pool, so we would have the normal pool (so the nimble_pool Ch version wouldn't need to change to support Ecto sandboxes) with something extra (probably a custom sandbox module in ecto_ch) to track session_ids + test pids ownership.

The CI is failing for unrelated reasons, I'll check it later as well.

[pepicrft]

Hey @ruslandoga
Thanks a lot 🙏🏼. No rush with this one. On our side, not being able to use transactions in our tests got a bit annoying, so I thought I'd take the lead in changing that so that we can run our tests in parallel. Let me know if I can help in any way.