feat(core, ui, localization): Add Poll attachment composer

[xsahil03x]

This pull request introduces several new features and improvements related to polls in the stream_chat package. Key changes include adding methods for managing polls in channels, updating poll-related models, and enhancing the user interface for creating and displaying polls.

Poll Management in Channels

• Added methods in Channel class to send, update, and delete polls. (packages/stream_chat/lib/src/client/channel.dart)

Poll Model Updates

• Introduced _NullConst class for handling nullable fields in Poll and PollOption classes. (packages/stream_chat/lib/src/core/models/poll.dart, packages/stream_chat/lib/src/core/models/poll_option.dart) [1] [2]
• Changed default value of enforceUniqueVote to true in Poll class. (packages/stream_chat/lib/src/core/models/poll.dart, packages/stream_chat/lib/src/core/models/poll.g.dart) [1] [2]
• Updated toJson methods to exclude null values for id fields in PollOption and PollVote classes. (packages/stream_chat/lib/src/core/models/poll_option.g.dart, packages/stream_chat/lib/src/core/models/poll_vote.g.dart) [1] [2]

User Interface Enhancements

• Added StreamPollCreator widget for creating polls within the message input attachment picker. (packages/stream_chat_flutter/lib/src/message_input/attachment_picker/options/stream_poll_creator.dart)
• Enhanced StreamUnreadIndicator to use Badge widget for displaying unread counts. (packages/stream_chat_flutter/lib/src/indicators/unread_indicator.dart)

Localization and Permission Updates

• Added multiple new localization strings related to poll creation and validation. (packages/stream_chat_flutter/lib/src/localization/translations.dart) [1] [2]
• Introduced a new permission type sendPoll in PermissionType class. (packages/stream_chat/lib/src/permission_type.dart)

[codecov]

Codecov Report

Attention: Patch coverage is 70.40905% with 340 lines in your changes missing coverage. Please review.

Project coverage is 61.04%. Comparing base (d2d5ac9) to head (395d6ec).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...chat_flutter/lib/src/theme/poll_creator_theme.dart	21.84%	93 Missing ⚠️
...ut/attachment_picker/stream_attachment_picker.dart	1.47%	67 Missing ⚠️
...ib/src/poll/poll_option_reorderable_list_view.dart	59.22%	42 Missing ⚠️
...er/lib/src/message_input/stream_message_input.dart	0.00%	29 Missing ⚠️
...attachment_picker/options/stream_poll_creator.dart	0.00%	23 Missing ⚠️
...utter/lib/src/poll/stream_poll_creator_dialog.dart	77.41%	21 Missing ⚠️
packages/stream_chat/lib/src/client/channel.dart	0.00%	13 Missing ⚠️
.../lib/src/misc/separated_reorderable_list_view.dart	58.33%	10 Missing ⚠️
...hat_flutter/lib/src/localization/translations.dart	73.91%	6 Missing ⚠️
..._picker/stream_attachment_picker_bottom_sheet.dart	0.00%	6 Missing ⚠️
... and 11 more

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2048      +/-   ##
==========================================
+ Coverage   60.30%   61.04%   +0.73%     
==========================================
  Files         326      336      +10     
  Lines       19074    20152    +1078     
==========================================
+ Hits        11502    12301     +799     
- Misses       7572     7851     +279     

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[deven98]

LGTM