chore: added visible region in change camera position callback for custom clusterization

[samirakhmedov]

Dependence on other Pull Requests (optional)

14

Background (optional)

This addition to functionality has to be added for custom clusterization to be possible, so then we can add it to our project

Changes

Added visible region to onCameraPositionChanged callback, so we always have up-to-date

Also implemented the example clusterization (has some pros when comparing with out-of-the-box cluster implementation)

https://github.com/surfstudio/yandex-mapkit-lite-flutter/assets/54618146/223d9c8b-98e1-454a-b085-6c1e900262bf

Checklist for self-check

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• [x] Specified Changes.
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• [x] All unspecified fields in the PR description deleted.
• [ ] New code covered by tests.

Checklist for reviewers

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Tests:

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[samirakhmedov]

@maxim-sysoev @internetova could you please review this PR and format it (i dont know what your format settings are), i really need this functionality for one of our projects

[dzmitry-struk-surf]

@internetova @Sadhorsephile @maxim-sysoev what stops you from reviewing the pr, are there any problems or blockers? If so, leave some feedback in comments, please.