rpc: Set RPCArg options with designated initializers
Summary:
For optional constructor arguments, use a new struct. This comes with two benefits:
Earlier unused optional arguments can be omitted
Designated initializers can be usedBackport of core#26074.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, PiRK
Reviewed By: #bitcoin_abc, PiRK
Subscribers: PiRK
Differential Revision: https://reviews.bitcoinabc.org/D16246