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 #183
• implement initialization/shutdown: open-feature/spec@a4ffec3 #184
• Implement event support  #186

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

[Kavindu-Dodan]

@toddbaert I will start working on the sub task: implement named client support (open-feature/spec@4cf8229)

[Kavindu-Dodan]

I will continue to work on implement initialization/shutdown: open-feature/spec@a4ffec3

[toddbaert]

@Kavindu-Dodan I've created sub-issues and assigned to you. Thanks!

[Kavindu-Dodan]

All functional and non-functional goals have been met with the linked PRs.