move-onlyish: break out CreateUTXOSnapshot from dumptxoutset

Summary:
This move/refactor is needed to set up a decent unittest for UTXO snapshot activation.

This is a backport of core#19806 [1/8]
https://github.com/bitcoin/bitcoin/pull/19806/commits/6606a4f8c616cf256537c3bfbdade9b43c51b4f5

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11224