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 https://github.com/auth0/auth0-flutter/issues/369

🎯 Testing

This was tested manually on the Quickstart sample app.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

```diff @@ 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](https://app.codecov.io/gh/auth0/auth0-flutter/pull/372/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | Coverage Δ | | |---|---|---| | [auth0_flutter](https://app.codecov.io/gh/auth0/auth0-flutter/pull/372/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `100.00% <100.00%> (ø)` | | | [auth0_flutter_android](https://app.codecov.io/gh/auth0/auth0-flutter/pull/372/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `97.35% <ø> (+0.79%)` | :arrow_up: | | [auth0_flutter_ios](https://app.codecov.io/gh/auth0/auth0-flutter/pull/372/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `99.84% <ø> (ø)` | | | [auth0_flutter_platform_interface](https://app.codecov.io/gh/auth0/auth0-flutter/pull/372/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0) | `87.03% <100.00%> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=auth0#carryforward-flags-in-the-pull-request-comment) to find out more.

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