update Bob's monero endpoint to use `--wallet-dir`, add `open_wallet` and `close_wallet` wallet-rpc calls
- use `open_wallet` to open wallet w/ funds instead: https://www.getmonero.org/resources/developer-guides/wallet-rpc.html#open_wallet
- also this allows us to make checks that the view-only wallet can be created upon receipt of the counterparty's view keys (for both parties)
- can use `close_wallet` after to close the view only on Alice's side to create the spend wallet after (or `stop_wallet`)?