Add proposal for Provider Hook

005-OFEP-provider-hook.md

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+## 005-OFEP-provider-hook.md
+
+## State: APPROVED
+
+As a part of the Provider implementation by the Flag Management Systen, there is a need to have standardized support for hooks specific to the Provider. The 
+Provider Hook(s) should be specific to the provider implementation and implement the various defined stages of the Hook interface as applicable to the 
+specific provider. 
+
+## Assumptions
+
+The Provider Hook is transparent to the application developer. The Provider may choose to document certain default behavior/feature in specific stages of the 
+Provider Hook as relevant to the application and the application developer may be allowed to override the default feature. 
+
+## Example Use Case
+
+Flag Management System X provides the option to add Experimentation as an extension of Feature Flag. The user creates a Feature Flag, runs basic on/off 
+evaluations of the Feature targeting specific audience and then wants to run A/B tests and hence adds the Experimentation aspects to the Feature Flag.
+
+The Provider needs to generate event specific to the Experimentation implementation  on successful flag evaluation. The system wants to use a Provider Hook 
+to do the following for each evaluation  
+1. Run initialization operations (Use the _before_ stage of the Hook)
+2. Run an event publish operation on successful flag evaluation operation (Use the _after_ stage of the Hook)
+3. Provide the option to the application developer to override the event publish operation 
+   1. The application developer may choose not to immediately publish the event on evaluation, but decide to publish it (or not) later based on other 
+      application specific logic
+
+For applications not required to override the default behavior, there is no extra step needed to add any other hooks. The Provider Hook is transparent to 
+the application developer.
+
+## Example Workflow
+
+1. At the time of registering the provider (OpenFeature.setProvider), Provider implementation can register one or more Provider Hook(s)
+2. Provider Hook implements the _before_ and _after_ and any other relevant stages to run the relevant provider logic.
+3. Provider documents the relevant default behavior (e.g. automatic publish of Experimentation event on successful evaluation)
+4. Provider provides an implementation of an Invocation Hook to override this default behavior as a part of the Provider SDK. 
+5. If needed, the application developer uses the invocation hook to override specific default behavior of the provider by using Hook Hints e.g. turn off 
+   default event publish behavior
+
+## Hook Ordering
+
+The Hooks should be executed in specific order.
+
+* before: API, Client, Invocation, Provider
+  * Provider does initialization after Invocation
+* after: Provider, Invocation, Client, API  
+* error (if applicable): Provider, Invocation, Client, API
+* finally: Provider, Invocation, Client, API
+