feat(ui): mark unread feature - UI

[Brazol]

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CLA

• [x] I have signed the Stream CLA (required).
• [x] The code changes follow best practices
• [x] Code changes are tested (add some information if not applicable)

Description of the pull request

The UI part of mark as unread feature with "Mark Unread" action and unread indicator

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 61.53846% with 70 lines in your changes are missing coverage. Please review.

Project coverage is 59.84%. Comparing base (305986c) to head (47a92e2).

Files	Patch %	Lines
...r/lib/src/message_list_view/message_list_view.dart	41.83%	57 Missing :warning:
...flutter/lib/src/message_widget/message_widget.dart	0.00%	13 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #1881 +/- ## ========================================== + Coverage 59.28% 59.84% +0.55% ========================================== Files 310 311 +1 Lines 17889 18036 +147 ========================================== + Hits 10605 10793 +188 + Misses 7284 7243 -41 ```

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