af: Install Kover from the library's Gradle config

[Widcket]

• All new/changed/fixed functionality is covered by tests (or N/A)
• I have added documentation for all new/changed functionality (or N/A)

📋 Changes

This PR makes sure the Kover Gradle plugin is properly installed on the library's Gradle config. Previously, it was installed on the example app's config and applied on the library's config. When installed on another app, it couldn't find the plugin.

📎 References

Fixes #369

🎯 Testing

This was tested manually on the Quickstart sample app.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (253209a) 96.08% compared to head (8795285) 96.31%.
Report is 33 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #372      +/-   ##
============================================
+ Coverage     96.08%   96.31%   +0.22%     
============================================
  Files            97       98       +1     
  Lines          1611     1573      -38     
  Branches        331      328       -3     
============================================
- Hits           1548     1515      -33     
- Misses           49       50       +1     
+ Partials         14        8       -6     

Flag	Coverage Δ
auth0_flutter	100.00% <100.00%> (ø)
auth0_flutter_android	97.35% <ø> (+0.79%)	⬆️
auth0_flutter_ios	99.84% <ø> (ø)
auth0_flutter_platform_interface	87.03% <100.00%> (ø)

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