[Chronik] Add block_tx_num_range to TxReader

Summary: Allows us to get the tx_nums of a block. Since all tx_nums of a block are a contiguous range of numbers, we return a Range.

Test Plan: ninja check-crates

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13792