validation: Move LoadExternalBlockFile to CChainState

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

LoadExternalBlockFile mainly acts on CChainState.

This is a backport of [[https://github.com/bitcoin/bitcoin/pull/20749 | core#20749]] [13/17]
https://github.com/bitcoin/bitcoin/pull/20749/commits/e0dc3057277c9576ddbfb8541599db0149e08bb6

Depends on D11184

Test Plan: ninja all check-all bitcoin-fuzzers

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11185