Add workhorse circuit breaker feature flag

What does this MR do and why?

Since feature flags are controlled by rails, we can return the flags value in a response for workhorse to use. The feature flag can toggle whether the circuit breaker is able to trip or not.

Also fixes a bug where the delegate roundTrip body was prematurely closed.

References

Related to https://gitlab.com/gitlab-org/gitlab/-/issues/525372

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