[Chronik] Add size to ffi::Block

Summary:
Allows us to store the size of the block for block stats.

CBlockIndex already has the size ready, so we don't need to do anything fancy.

Test Plan: ninja test_bitcoin && ./src/test/test_bitcoin -t bridgeprimitives_tests

Reviewers: Fabien, #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D13850