validation: add CChainState.m_from_snapshot_blockhash

Summary:

This parameter is unused, but in future commits will allow
ChainstateManager to differentiate between chainstates created from a
UTXO snapshot from those that weren't.

Partial backport (1/5) of core PR17737:
https://github.com/bitcoin/bitcoin/pull/17737/commits/8e2ecfe2496d8a015f3ee8723025a438feffbd28

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D8537