wallet: Split signmessage from rpcwallet

Summary:

rpcwallet is the file that takes longest to compile, especially with sanitizers enabled it can take several 10s of seconds.

Allow faster incremental and parallel builds by starting to split it up. First, split off signmessage, which is unrelated to other stuff such as wallet file handling, wallet encryption, tx creation, or wallet status/info.

This is a backport of core#23602

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12774