Partial backport of Core PR9681 to add CWallet::SignTransaction

Summary:
Partial backport (only wallet.* changes in PR9681)
https://github.com/bitcoin/bitcoin/pull/9681/files
Completes T547
Although the function is currently unused, it appears to be used in future backports.
We can remove it later if it proves not useful.

Test Plan: ninja check

Reviewers: deadalnix, Fabien, #bitcoin_abc

Reviewed By: deadalnix, #bitcoin_abc

Subscribers: teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2675