monero: Use fee priority enums from monero repo CLI/RPC wallets (#499)

* monero: Use fee priority enums from monero repo CLI/RPC wallets

* Update processor for fee priority change

* Remove FeePriority::Default

Done in consultation with @j-berman.

The RPC/CLI/GUI almost always adjust up except barring very explicit commands,
hence why FeePriority 0 is now only exposed via the explicit command of
FeePriority::Custom { priority: 0 }.

Also helps with terminology.

---------

Co-authored-by: Luke Parker <lukeparker5132@gmail.com>

tests/full-stack/src/tests/mint_and_burn.rs

@@ -389,7 +389,7 @@ async fn mint_and_burn_test() {
         )],
         &Change::new(&view_pair, false),
         vec![Shorthand::transfer(None, serai_addr).encode()],
-        rpc.get_fee(Protocol::v16, FeePriority::Low).await.unwrap(),
+        rpc.get_fee(Protocol::v16, FeePriority::Unimportant).await.unwrap(),
       )
       .unwrap()
       .sign(&mut OsRng, &Zeroizing::new(Scalar::ONE))