[Chronik] Add BlockReader to read blocks from Db
Summary:
Methods:
- height for current db block height.
- tip for latest block.
- by_height for block by height.
Depends on D11798
Test Plan: ninja check-crates
Reviewers: Fabien, #bitcoin_abc
Reviewed By: Fabien, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D11799