Changes to support spec v0.6.0 compliance

[toddbaert]

Implement specification v0.6.0 changes

See changes since last release.

Please note there's no need to complete all these tasks in a single PR (in fact, that's not recommended).

Functional requirements:

• implement flag metadata: open-feature/spec@74c373e
• implement named client support: open-feature/spec@4cf8229 #442
• Implement Initialize/Shutdown on provider registration #449 : open-feature/spec@a4ffec3
• Implement events #440
  • the spec is intentionally loose here to allow for an implementation that feels "java-ish", consider:
    • java.util.EventListener and its subclasses
    • a custom interface
    • other options?

Non-functional requirements:

• add relevant tests
• add examples and documentation, see template README
• update specification badge to v0.6.0:

Keep in mind:

• backwards compatibility is highly encouraged
  • take every precaution to ensure existing code written by application authors and existing providers implementations, do not break
• it's recommended these features are released together

[toddbaert]

named client support was implemented by @justinabrahms here.

[toddbaert]

I'm interested in taking on the events part of this. I'm going to open a new issue specifically about that if nobody objects.

[Kavindu-Dodan]

I am actively working on metadata requirement and will open a PR soon for the review

[toddbaert]

Done with #364