Move additional_key.rs to serai-processor-view-keys

I don't love this. I wanted to simply add this function to `processor/key-gen`,
but then anyone who wants a view key needs to pull in Bulletproofs which is a
mess of code. They'd also be subject to an AGPL licensed library.

This is so small it should be a primitive elsewhere, yet there is no primitives
library eligible. Maybe serai-client since that has the code to make
transactions to Serai (and will have this as a dependency)? Except then the
processor has to import serai-client when this rewrite removed it as a
dependency.

processor/src/main.rs

@@ -27,9 +27,6 @@ use serai_env as env;
 
 use message_queue::{Service, client::MessageQueue};
 
-mod plan;
-pub use plan::*;
-
 mod networks;
 use networks::{Block, Network};
 #[cfg(feature = "bitcoin")]
@@ -39,9 +36,6 @@ use networks::Ethereum;
 #[cfg(feature = "monero")]
 use networks::Monero;
 
-mod additional_key;
-pub use additional_key::additional_key;
-
 mod db;
 pub use db::*;