Currently the RPC method implementations have many issues:
- Default RPC argument values (and their optionality state) are duplicated in the documentation and the C++ code, with no checks to prevent them from going out of sync.
- Getting an optional RPC argument is verbose, using a ternary operator, or worse, a multi-line if.
Fix all issues by adding default helper that can be called via self.Arg<int>(0). The helper needs a few lines of code in the src/rpc/util.h header file. Everything else will be implemented in the cpp file once and if an RPC method needs it.
There is also an self.MaybeArg<int>(0) helper that works on any arg to return the argument, the default, or a falsy value
This is a backport of core#28230
Depends on D17943