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

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: https://github.com/open-feature/spec/commit/74c373e089ad77bf8cac84f3d93c00c945ff3a8a
• implement named client support: https://github.com/open-feature/spec/commit/4cf8229d23a88feeaadf8b066e5d3662b94ff262
• implement initialization/shutdown: https://github.com/open-feature/spec/commit/a4ffec3d4416b8ad6d12d1ebfa2faf81ad153f65
• implement events: https://github.com/open-feature/spec/commit/bf9e45b1d51c316ee2d6340bc2b807294995d0ad

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

Start by reading the linked specification comparison and the four referenced commits for the v0.7.0 requirements. Use the template README when planning examples and documentation, and add relevant tests. Done means the listed metadata, named clients, initialization/shutdown, and events requirements are addressed, the badge is updated, and backwards compatibility is preserved.