[Chronik] Add Node, a wrapper struct to access the node from Chronik

Summary:
In order to broadcast txs from Chronik, we need to be able to talk to the node via ChronikBridge.

For this, we add Node, which contains a UniquePtr to the bridge, and add it as an Extension layer to the HTTP router. This way we can access it from HTTP.

Also change Chronik to use Node, so we can re-use the same struct.

Test Plan: ninja check-crates

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13987