Put Job Router discovery endpoint behind feature flag

What does this MR do and why?

Put Job Router discovery endpoint behind feature flag

This change set puts the new job router discovery endpoint behind a feature flag to better control the roll out of the feature flag.

There already exists a feature flag on the Runner, but we need a way to disable the Job Router entirely on an instance level.

There are two feature flags:

• job_router_instance_runners: enable job router for instance runners only.
• job_router: enable job router for top-level groups or globally.

Full context: &19607 (comment 2937165722)

References

Closes #583473

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