Support SpEL expressions in the @Region annotation

Another option to consider with regards to SGF-261 - implementing the "ability to persist application domain objects (entities) to multiple Regions in a GemFire Cache" might be to allow SpEL expressions in the @Region annotation's value attribute enabling developers to customize the Region that an entity is persisted to on a call by call basis (a thread bound property or the like). MongoDB already supports that, see BasicMongoPersistentEntity, for example.