Move circuit breaker code to Gitlab::CircuitBreaker namespace

As a part of the Introduce circuit breaker for OpenAI client (!117993 - merged) note, we want to move the ee/lib/gitlab/llm/concerns/circuit_breaker.rb outside LLM namespace.

Question (non-blocking) This functionality seems pretty generic. Thoughts on moving it outside of LLM namespace later (hence non-blocking)? 🤔