validation: Pass in blockman to ContextualCheckBlockHeader

Summary:
[META] This commit should be followed up by removing the comments and assertions meant only to show that the change is correct.

This is a backport of core#20749 [8/17]
https://github.com/bitcoin/bitcoin/pull/20749/commits/d363d06bf7d6c3736140672ba8a7f82f4d6fb6ab

Depends on D11176

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11177