[FEATURE] Make provider interface "stateless", SDK maintains provider state

[toddbaert]

Stateless providers

Providers no longer maintain their own state: the state for each provider is maintained in the SDK automatically, and updated according to the success/failures of lifecycle methods (init/shutdown) or events emitted from providers spontaneously.

Tasks

Beta Give feedback

1. Requirement 1.7.1: The client MUST define a provider status accessor which indicates the readiness of the associated provider, with possible values NOT_READY, READY, STALE, ERROR, or FATAL.
2. Requirement 1.7.3: The client's provider status accessor MUST indicate READY if the initialize function of the associated provider terminates normally.
3. Requirement 1.7.4: The client's provider status accessor MUST indicate ERROR if the initialize function of the associated provider terminates abnormally.
4. Requirement 1.7.5: The client's provider status accessor MUST indicate FATAL if the initialize function of the associated provider terminates abnormally and indicates error code PROVIDER_FATAL.
5. Requirement 1.7.6: The client MUST default, run error hooks, and indicate an error if flag resolution is attempted while the provider is in NOT_READY.
6. Requirement 1.7.7: The client MUST default, run error hooks, and indicate an error if flag resolution is attempted while the provider is in FATAL.
7. Requirement 1.7.8: Implementations SHOULD propagate the error code returned from any provider lifecycle methods.
8. Requirement 5.1.5: PROVIDER_ERROR events SHOULD populate the provider event details's error code field.
9. Requirement 5.3.5: If the provider emits an event, the value of the client's provider status MUST be updated accordingly.