[backport#16244] Move wallet creation out of the createwallet rpc into its own function

Summary:
https://github.com/bitcoin/bitcoin/pull/16244/commits/1aecdf2063cbe28d4715ae5ae1a7e51b860c9f4d

Depends on D6423

Backport of Core PR16244

Test Plan:

ninja check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D6425