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