Start on the new processor main loop

processor/bitcoin/src/db.rs

@@ -1,5 +1,13 @@
+use serai_client::validator_sets::primitives::Session;
+
 use serai_db::{Get, DbTxn, create_db};
 
+create_db! {
+  Processor {
+    ExternalKeyForSession: (session: Session) -> Vec<u8>,
+  }
+}
+
 create_db! {
   BitcoinProcessor {
     LatestBlockToYieldAsFinalized: () -> u64,