Changes to support spec v0.6.0 compliance

[toddbaert]

⚠️ 0.6.0 should probably be undertaken only after the existing 1.0-functional requirements are complete.

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). Consider creating a new sub issue for each of the below requirements.

Functional requirements:

• implement flag metadata: open-feature/spec@74c373e
  feat: pass flag_metadata from resolution to evaluation details #212
• implement named client support
• Implement Initialize/Shutdown on provider registration: open-feature/spec@a4ffec3
  feat: implement initialize/shutdown on provider registration #213
• Implement events https://github.com/open-feature/spec/blob/main/specification/sections/05-events.md
  • Implement API-Level Events #131 (API level events)

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