Allow metadata hint values to be contributed by multiple components

[snicoll]

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.

[snicoll]

We've discussed the possibility to add an extra attribute to hint to indicate what should happen when multiple entries are to be found for the same key. There are probably other use cases where a single key could be contributed by more than one resource so we need to consider all use cases before introducing that change.