feat!: Spec v0.8 adherence

[fabriziodemaria]

Spec v0.8 adherence

• Created "ProviderStatus.swift" according to Specification.
  • ProviderState is now maintained by the SDK, and not by the Provider implementations
• Align "ProviderEvent.swift" with Specification
  • Basic events like Ready, Error or Fatal are emitted by the SDK and are not expected by the Provider implementations.
• Makes initialize and onContextChange protocols throwing, allowing for the OpenFeatureAPI layer to detect cases where Error events should be emitted
• Makes initialize and onContextChange protocols async for an easier Provider implementation (in most cases)

Missing parts

• From the Specification: It's possible that the on context changed function is invoked simultaneously or in quick succession; in this case the SDK will only run the PROVIDER_CONTEXT_CHANGED handlers after all reentrant invocations have terminated, and the last to terminate was successful (terminated normally)
  • This is not part of this PR: subsequent calls are serialized and executed in sequence, each returning from its execution even if more same functions are queued
• Handling of spontaneous Provider events missing in this PR

Changes unrelated to Spec v0.8

• Added setEvaluationContextAndWait to provide the same better ergonomics that setProviderAndWait already offers
• Evaluation returns early if provider state is not "ready"
• Refactor afterAll hook to adhere to latest conventions

Notes on backwards compatibility

• A backwards incompatible aspect of this change is that the initialize() implementation needs to throw in case of errors, rather than emitting the ERROR event. The latter is now responsibility of the SDK. This is also valid for onContextChange.
• This change in semantics is not enforced with code, and it might be easy for the Provider implementer to make mistakes when adopting this new version of the SDK (e.g. forget to remove ERROR emission from the Provider side)

Example Adoption

The Confidence OpenFeature Provider adopting this version of the SDK is in the works here: spotify/confidence-sdk-swift#184

[beeme1mr]

Hey @fabriziodemaria, the change looks good from what I can tell (limited experience with Swift). Since this is a breaking change, it may be a good time to update the afterAll stage in the hooks to comply with a recent update to the spec.

https://openfeature.dev/specification/sections/hooks#requirement-438

This will make it easy to support more sophisticated telemetry use cases.

[beeme1mr]

@nicklasl, any concerns with the PR?