replace asserts in RPC code with CHECK_NONFATAL and add linter (Adam Jonas)

Pull request description:

• Replace instances of assert in /rpc files and rpcwallet with CHECK_NONFATAL(condition)
• Add a linter to prevent future usage of assert being used in RPC code
  
  ref https://github.com/bitcoin/bitcoin/pull/17192

Depends on D7402

Backport of Core PR17318

ninja check check-functional

For the linter, change a CHECK_NONFATAL back to assertion on a file inside the src/rpc diretory, an rpcXXXXX.cpp file in src/wallet and another file in the latter directory whose filename _does not_ start with 'rpc'

only the first two should trigger the new linter's error