fix(ui): custom attachment builders

[deven98]

PR based on #1826

The attachment builder now expects custom attachments only and adds the attachment builders for the normal attachment types by default.

Old:

messageWidget.copyWith(
  // ....
  attachmentBuilders: [
    CustomBuilder()
    ...StreamAttachmentWidgetBuilder.defaultBuilders(message: messageDetails.message),
  ],
);

New:

messageWidget.copyWith(
  // ....
  attachmentBuilders: [
    CustomBuilder()
  ],
);

OR

messageWidget.copyWith(
  // ....
  attachmentBuilders: [
    ...StreamAttachmentWidgetBuilder.defaultBuilders(
      message: messageDetails.message,
        customAttachmentBuilders: [
           CustomBuilder(),
        ],
     ),
  ],
);

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 60.28%. Comparing base (5ea210e) to head (e851030).

Files	Patch %	Lines
...tter/lib/src/message_widget/parse_attachments.dart	0.00%	2 Missing ⚠️
.../attachment/builder/attachment_widget_builder.dart	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1938      +/-   ##
==========================================
- Coverage   60.28%   60.28%   -0.01%     
==========================================
  Files         317      317              
  Lines       18478    18479       +1     
==========================================
  Hits        11140    11140              
- Misses       7338     7339       +1     

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[Brazol]

LGTM