This change adds an optional wallet=filename argument to RPCs that access the wallet so they can operate on wallets other than the default wallet.
This does essentially the same thing as #10653, except it adds support for new wallet parameters explicitly in individual RPC methods instead of implicitly at the RPC dispatching layer.
Example usage:
bitcoind -regtest -wallet=w1.dat -wallet=w2.dat
bitcoin-cli -regtest -named getwalletinfo wallet=w1.dat
bitcoin-cli -regtest -named getwalletinfo wallet=w2.dat
bitcoin-cli -regtest -named getbalance wallet=w2.dat