feat(ui): mark unread feature - UI

[Brazol]

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Description of the pull request

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

[esarbanis]

We need to add the new components in our documentation

[esarbanis]

LGTM

[codecov]

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 ⚠️
...flutter/lib/src/message_widget/message_widget.dart	0.00%	13 Missing ⚠️

Additional details and impacted files

@@            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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[esarbanis]

LGTM