Smash out the signer

Abstract, to be done for the transactions, the batches, the cosigns, the slash
reports, everything. It has a minimal API itself, intending to be as clear as
possible.

.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-frost-attempt-manager \
             -p serai-processor \
             -p tendermint-machine \
             -p tributary-chain \