feat: isolate interfaces from SDK to improve testability

[Kavindu-Dodan]

This PR

Partially address #266 by providing interface contracts for the SDK. There are no breaking changes in this change

I have introduced IEvaluation interface, which represents the general OpenFeatuer API layer [1]. Before this change, there was no such interface so mocking API interactions was impossible. To obtain the singleton instance of IEvaluation interface, I have introduced GetApiInstance() helper.

Consider the example below,

// get the global instance that may be injected into testable components 
apiInstance := openfeature.GetApiInstance()

// use for OpenFeature operations 
apiInstance.SetProvider(myProvider)
apiInstance.AddHandler(....)

In the future, we can update our documentation to make API instance based operations the preferred interaction method.

Further, IEvaluation exposes client creation methods GetClient() & GetNamedClient(clientName string) (similar to spec [2]). These methods return IClient instances, making it easy to test OpenFeature client related operations.

apiInstance := openfeature.GetApiInstance()

// IClient instance, which can be mocked when injected into preferred locations 
client := apiInstance.GetClient()

[1] - https://openfeature.dev/specification/sections/flag-evaluation
[2] - https://openfeature.dev/specification/sections/flag-evaluation#creating-clients

[codecov]

Codecov Report

Attention: Patch coverage is 91.66667% with 9 lines in your changes are missing coverage. Please review.

Project coverage is 81.92%. Comparing base (a9e19dd) to head (30e3e4e).

Files	Patch %	Lines
openfeature/client.go	72.22%	5 Missing ⚠️
openfeature/openfeature_api.go	93.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #268      +/-   ##
==========================================
+ Coverage   81.74%   81.92%   +0.17%     
==========================================
  Files          10       10              
  Lines         964      979      +15     
==========================================
+ Hits          788      802      +14     
  Misses        156      156              
- Partials       20       21       +1     

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[hairyhenderson]

I'm not clear on why this intermediate interface is necessary - can you clarify why it exists?

[Kavindu-Dodan]

I introduced this to support internal work and avoid direct access to singleton instance from other components like Client. For example, a client instance needs to obtain the provider associate with its domain, so it rely on ForEvaluation method exposed by this internal interface.

[hairyhenderson]

maybe name this AddClientHandler to reflect the relationship to AddHandler (i.e. the same except scoped), or perhaps rename AddHandler above as RegisterHandler?

[Kavindu-Dodan]

will rename to AddClientHandler. Regarding AddHandler, this is already public so I will avoid a breaking change and keep it there.