Add a public TransactionKeys struct to monero-wallet

monero-wallet ships an Eventuality, yet it's across the entire transaction. It
can't prove a single output's state with a traditional payment proof. By adding
this new object, another library can obtain the ephemeral randomness used and
do any/every proof they want regarding a transaction's outputs.

Necessary for https://github.com/serai-dex/serai/issues/599.

networks/monero/wallet/src/send/mod.rs

@@ -29,6 +29,7 @@ use crate::{
 };
 
 mod tx_keys;
+pub use tx_keys::TransactionKeys;
 mod tx;
 mod eventuality;
 pub use eventuality::Eventuality;