rpc: Extract GetWalletNameFromJSONRPCRequest from GetWalletForJSONRPCRequest

Summary:
Splits GetWalletForJSONRPCRequest() into two functions.

Refactor necessary for next part of this backport.

Partial backport of Core PR13111
https://github.com/bitcoin/bitcoin/pull/13111/commits/537efe19e696a97ca6a4f461b2c1b4cb7ab001b7

Test Plan:

make check
test_runner.py

Reviewers: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Reviewed By: deadalnix, Fabien, jasonbcox, O1 Bitcoin ABC, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D4252