Changes to support spec v0.6.0, v0.7.0 compliance

[toddbaert]

Implement specification v0.7.0 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
• implement initialization/shutdown: open-feature/spec@a4ffec3
• implement events: open-feature/spec@bf9e45b

Non-functional requirements:

• add relevant tests
• add examples and documentation, see template README
• update specification badge to v0.7.0:

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