Allow accessing declaring Bean from ObserverMethod

CDI issue - https://github.com/eclipse-ee4j/cdi/issues/563

CDI 4 allows to accessing declaring Bean object from ObserverMethod.
Weld already has internal construct for this, we just need to check if we expose it accordingly. See this code.

We also need test coverage for two cases:

• Standard, class-based bean where this method should return the declaring bean
• Synthetic OM in which case we (if memory serves) return the extension declaring it.
  • Note that in this scenario, the spec leaves it undefined (so no TCKs for it)