This adds a /wallet/* endpoint to the RPC server (optional).
The wallet RPC functions will try to find the selected wallet by comparing the given endpoint with all available wallets/walletIDs.
The walletID is currently defined by the filename (-wallet=<filename>), ideally, we later add a wallet RPC call to set the walletID (should be written to the wallet database).
This also includes endpoint support for bitcoin-cli.
There is a new argument -wallet=<walletID>. If set, the given walletID will be used to call the endpoint with a scheme of /wallet/< walletID >.
QA's authproxy is also extended to allow calls like self.nodes[0].<walletID>.<method>.
There is finally also a basic multiwallet RPC test.
Contains #10649.