RPC: Extract InvalidateBlock and ReconsiderBlock helpers

Summary:
The end goal is to reuse these helpers in dumptxoutset when dumping a utxo snapshot for a particular block height.

This is a partial backport of core#29553

https://github.com/bitcoin/bitcoin/pull/29553/commits/446ce51c21cd2466cb12fa0166fd069d42b603bf
https://github.com/bitcoin/bitcoin/pull/29553/commits/fccf4f91d21c351d742943d35476f53d40963b8b

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17998