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.

.github/workflows/tests.yml

@@ -40,6 +40,7 @@ jobs:
             -p serai-message-queue \
             -p serai-processor-messages \
             -p serai-processor-key-gen \
+            -p serai-processor-view-keys \
             -p serai-processor-frost-attempt-manager \
             -p serai-processor-primitives \
             -p serai-processor-scanner \