[backport#17304 10/18] Refactor: Move LoadKey LegacyScriptPubKeyMan method definition

Summary:
This commit does not change behavior.

https://github.com/bitcoin/bitcoin/pull/17304/commits/9716bbe0f8081814cbf052e8211d1c6a838e5262

Depends on D7156

Partial backport of Core PR17304

Test Plan:

ninja check

Reviewers: #bitcoin_abc, jasonbcox

Reviewed By: #bitcoin_abc, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7157