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.

Cargo.toml

@@ -71,6 +71,7 @@ members = [
 
   "processor/messages",
   "processor/key-gen",
+  "processor/view-keys",
   "processor/frost-attempt-manager",
 
   "processor/primitives",