Allow metadata hint values to be contributed by multiple components

While reviewing #18363 we concluded that endpoint values could be contributed from multiple different components and offering auto-completion only for the core components could be misleading.

We should improve the meta-data format to be able to:

• Have the same list of values for multiple keys without having to define it multiple times. Endpoint IDs can be used in includes, excludes, web mappings, etc. Right now the full list has to be defined as a metadata hint for every key
• Offer a way for another component (jar) to contribute additional value (if allowed). This would allow other modules defining Actuator endpoints to provide their own values

Given that the annotation processor has already some logic for actuator endpoint, we could generate the latter automatically but it is an interesting concept for other use cases too.

Start by examining the existing metadata format and the annotation-processor logic for Actuator endpoints mentioned in the issue. Trace how endpoint values are currently defined for metadata hints, then establish how shared values and contributions from other components should be represented and verified.