[backport#13756] wallet/rpc: add setwalletflag RPC and MUTABLE_WALLET_FLAGS

Summary:
https://github.com/bitcoin/bitcoin/pull/13756/commits/f904723e0d5883309cb0dd14b826bc45c5e776fb

Depends on D6463

Partial backport of Core PR13756

Test Plan:

ninja check

the proper functional test for this feature is included in a posterior commit D6466

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6464