Explicitly type our enriched signals

These are being typed implicitly and verified against SignalSourceHit[]
on the assignment below, but this makes the types explicit and surfaces
a type error here instead of the subsequent assignment.

x-pack/plugins/security_solution/server/lib/detection_engine/signals/threat_mapping/enrich_signal_threat_matches.ts

@@ -82,7 +82,7 @@ export const enrichSignalThreatMatches = async (
     buildMatchedIndicator({ queries, threats: matchedThreats })
   );
 
-  const enrichedSignals = uniqueHits.map((signalHit, i) => {
+  const enrichedSignals: SignalSourceHit[] = uniqueHits.map((signalHit, i) => {
     const threat = get(signalHit._source, 'threat') ?? {};
     if (!isObject(threat)) {
       throw new Error(`Expected threat field to be an object, but found: ${threat}`);