Implement the Entitlements API

[winsmith]

Implements these APIs:

• Feature
• Product Feature
• Active Entitlement

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 67 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@5c579be). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...StripeKit/Entitlements/Feature/FeatureRoutes.swift	0.00%	32 Missing ⚠️
...it/Entitlements/Feature/ProductFeatureRoutes.swift	0.00%	17 Missing ⚠️
...s/Active Entitlement/ActiveEntitlementRoutes.swift	0.00%	13 Missing ⚠️
Sources/StripeKit/StripeClient.swift	0.00%	5 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff           @@
##             main    #277   +/-   ##
======================================
  Coverage        ?   2.02%           
======================================
  Files           ?     197           
  Lines           ?   10355           
  Branches        ?       0           
======================================
  Hits            ?     210           
  Misses          ?   10145           
  Partials        ?       0           

Files with missing lines	Coverage Δ
Sources/StripeKit/StripeClient.swift	0.00% <0.00%> (ø)
...s/Active Entitlement/ActiveEntitlementRoutes.swift	0.00% <0.00%> (ø)
...it/Entitlements/Feature/ProductFeatureRoutes.swift	0.00% <0.00%> (ø)
...StripeKit/Entitlements/Feature/FeatureRoutes.swift	0.00% <0.00%> (ø)