rpc: Keep default argument value in correct type

Summary:
Store default values of RPC arguments in the corresponding type instead of a string. The value is then serialized when the help output is needed.

This enables checking default value type againts argument type.

This is a backport of core#21679
Depends on D14405

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14409