[Chronik] Move ffi::Block into chronik-bridge, use CBlockIndex and CBlock

Summary:

1. Move ffi::Block from chronik-lib to chronik-bridge. This allows us to use this type in other crates, e.g. chronik-indexer.
2. Add ffi definitions for CBlockIndex and CBlock so we can use them in Rust.
3. Expose BridgeBlock through ffi so we can use it in Rust, renamed to bridge_block.
4. Change handle_block_connected and handle_block_disconnected to take &CBlock and &CBlockIndex.

Test Plan: ninja check-crates && ninja && ./test/functional/test_runner chronik_block

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13276