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/multisigs/mod.rs

@@ -4,5 +4,5 @@ pub enum MultisigEvent<N: Network> {
   // Batches to publish
   Batches(Option<(<N::Curve as Ciphersuite>::G, <N::Curve as Ciphersuite>::G)>, Vec<Batch>),
   // Eventuality completion found on-chain
-  Completed(Vec<u8>, [u8; 32], <N::Eventu
+  Completed(Vec<u8>, [u8; 32], N::Eventuality),
 }