[service] Stabilize `service.AllowNoPipelines` feature gate

[mx-psi]

Component(s)

No response

Describe the issue you're reporting

#12613 introduces a feature gate that allows starting the collector without pipelines. There are some open questions as to how this should work after stabilization. Some options that have been discussed are:

1. Just allow the Collector to start without any pipelines configured. This has the downside that it's easier to misconfigure the Collector.
2. Allow the Collector to start without any pipelines configured, if there is a suitable extension configured (like OpAMP). This seems like a natural option if there is a natural way of classifying extensions as 'allowed' to do this.
3. Introduce a specific service::pipelines::enabled config option. This may be confusing if an external tool (again, OpAMP) configures pipelines.

I argue that this should be a blocker for moving this feature gate for beta

[yurishkuro]

What is the decision process? RFC? A vote?

[mx-psi]

For now I would want to discuss it on this issue and figure out if there is disagreement. If there is disagreement then an RFC would be my preference