Feature: decouple index DB from specific RPC implementation

Is your feature request related to a problem? Please describe. Today, electrs only supports JSONRPC, so adding more RPC implementations is tricky (e.g. https://github.com/romanz/electrs/issues/257).

Describe the solution you'd like We should probably decouple all index build/query code into a separate library crate, allowing composing it with specific RPC protocols.

Describe alternatives you've considered Using cargo features (similar to how BWT allows both Electrum RPC and HTTP API).