[FEATURE] Implement Tracking in PHP

[toddbaert]

Requirements

Implement tracking as per spec: open-feature/spec@cd99c35

Functional requirements:

• implement all the features described here

Non functional requirements:

• use as many existing abstractions and implementations as possible
  • ie: tracking event details should likely use similar underlying functionality to evaluation context / structure
• provider interface(s) are modified to optionally support tracking; not all providers will support it
• associated testing and README entry (see markdown snippet below)
• no breaking changes

README addition (be careful to correctly link to #hooks and #providers and update the code snippet for the language in question

### Tracking

The tracking API allows you to use OpenFeature abstractions and objects to associate user actions with feature flag evaluations.
This is essential for robust experimentation powered by feature flags.
For example, a flag enhancing the appearance of a UI component might drive user engagement to a new feature; to test this hypothesis, telemetry collected by a hook(#hooks) or provider(#providers) can be associated with telemetry reported in the client's `track` function.

{example-language-in-question}

Note that some providers may not support tracking; check the documentation for your provider for more information.