rpc: Auto-format RPCResult

Summary:
Backport of Core PR17809 part [2/2] : https://github.com/bitcoin/bitcoin/pull/17809/commits/fa6b061fc118995eec41766519a11bc0dd1a901d

Depends on D8123

Test Plan:

ninja all check-all

Examine the differential in documentation to ensure it is correct: https://reviews.bitcoinabc.org/differential/diff/25141/

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8124