Move txoutproof RPCs to txoutproof.cpp

Summary:
The txoutproof RPCs don't really fit into rawtransaction.cpp, as they deal with txids, not with raw transactions. As they are placed in the blockchain RPC category, they could be moved there. However, blockchain.cpp already takes about 20 seconds to compile (and rawtransaction.cpp even longer), so move them to a separate file.

Can be reviewed with --color-moved=dimmed-zebra --color-moved-ws=ignore-all-space

This is a backport of core#24582

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13051