fix: update provider object eval, T -> Structure

[toddbaert]

I neglected to change T -> Structure in the FeatureProvider interface (I only got the Features interface which corresponds to the evaluation API) in #51. T is so permissive, that it was easy to miss since there wasn't compilation errors.

We certainly want the type in the provider to match the evaluation API.

Since this just changes generics, I believe there is no runtime changes here.

[codecov-commenter]

Codecov Report

Merging #54 (3cf5a6f) into main (405e812) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               main      #54   +/-   ##
=========================================
  Coverage     89.60%   89.60%           
  Complexity      145      145           
=========================================
  Files            19       19           
  Lines           327      327           
  Branches          6        6           
=========================================
  Hits            293      293           
  Misses           25       25           
  Partials          9        9           

Flag	Coverage Δ
unittests	89.60% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
.../java/dev/openfeature/javasdk/FeatureProvider.java	100.00% <ø> (ø)
...ain/java/dev/openfeature/javasdk/NoOpProvider.java	96.55% <100.00%> (ø)
...ava/dev/openfeature/javasdk/OpenFeatureClient.java	97.53% <100.00%> (ø)

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