Remove g_rpc_chain global
Summary:
Replace with RPC request reference to new WalletContext struct similar to the existing NodeContext struct and reference. This PR is a followup to #18740 removing the g_rpc_node global. Some later PRs will follow this up and move more wallet globals to the WalletContext struct.
Backport of core PR19096.
Adapted to match our wallet dump RPCs separation.
Test Plan:
ninja all check-all
Reviewers: #bitcoin_abc, majcosta
Reviewed By: #bitcoin_abc, majcosta
Subscribers: majcosta
Differential Revision: https://reviews.bitcoinabc.org/D8586