Replace with RPC request reference to new WalletContext struct similar to the
existing NodeContext struct and reference.
This PR is a followup to 25ad2c623ahttps://github.com/bitcoin/bitcoin/pull/18740 removing the g_rpc_node global.
Some later PRs will follow this up and move more wallet globals to the
WalletContext struct.
Co-authored-by: João Barbosa <joao.paulo.barbosa@gmail.com>