Add scheduler-primitives

The main benefit is whatever scheduler is in use, we now have a single API to
receive TXs to sign (which is of value to the TX signer crate we'll inevitably
build).

.github/workflows/tests.yml

@@ -42,9 +42,10 @@ jobs:
             -p serai-processor-key-gen \
             -p serai-processor-frost-attempt-manager \
             -p serai-processor-primitives \
+            -p serai-processor-scanner \
+            -p serai-processor-scheduler-primitives \
             -p serai-processor-utxo-scheduler-primitives \
             -p serai-processor-transaction-chaining-scheduler \
-            -p serai-processor-scanner \
             -p serai-processor \
             -p tendermint-machine \
             -p tributary-chain \