add find references support for property keys in `@Value` annotation

[martinlippert]

Auto completion and go to definition support already exists for property keys in @Value annotations. We need to add support for find references (all references to the property key in the code, including @ConditionalOnProperty annotation prefix/values, property files, etc.)

The same logic should be re-used across the different annotations referring to properties (and probably even for find references logic in property files themselves as well).

[martinlippert]

Already implemented for a long time. Closing this.