rpc: Move OuterType enum to header

Summary:
This is needed so that it can be used by RPCResult

Also,

• rename NAMED_ARG to NONE for generalization.
• change RPCArg constructors to initialize the members by moving values

Backport of Core PR17809 [1/2] : https://github.com/bitcoin/bitcoin/pull/17809/commits/fa7d0503d320900e14c4d9bc016d65c7431070bb

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8123