Add ResourceExhaustedCause for namespace operations (#562)

**What changed?**
Added a new ResourceExhaustedCause
RESOURCE_EXHAUSTED_CAUSE_OPERATIONS_RATE_LIMIT.

**Why?**
We are introducing a new namespace operations rate limit which can
throttle frontendRPS, poll requests and history tasks. That rate limiter
will return this cause.

**Breaking changes**
No

Author: prathyushpv

temporal/api/enums/v1/failed_cause.proto

@@ -141,6 +141,8 @@ enum ResourceExhaustedCause {
     RESOURCE_EXHAUSTED_CAUSE_PERSISTENCE_STORAGE_LIMIT = 7;
     // Circuit breaker is open/half-open.
     RESOURCE_EXHAUSTED_CAUSE_CIRCUIT_BREAKER_OPEN = 8;
+    // Namespace exceeds operations rate limit.
+    RESOURCE_EXHAUSTED_CAUSE_OPS_LIMIT = 9;
 }
 
 enum ResourceExhaustedScope {