`CRPCTable::dumpArgMap` currently works by casting RPC command `unique_id` integer field to a function pointer, and then calling the function. The `unique_id` field wasn't supposed to be used this way (it's meant to be used to detect RPC aliases), and this code segfaults in the `rpc_help.py` test in multiprocess PR [[https://github.com/bitcoin/bitcoin/pull/10102 | core#10102]] because wallet RPC functions aren't directly accessible from the node process.
Fix this by adding a new `GET_ARGS` request mode to retrieve argument information similar to the way the `GET_HELP` mode retrieves help information.
This is a backport of [[https://github.com/bitcoin/bitcoin/pull/21035 | core#21035]] [3/3]
https://github.com/bitcoin/bitcoin/pull/21035/commits/9048c58e10841d9e1d709c0a325dd14684cec325