Allow configuring opacity for muted channel list item

[trentBowden]

Please select which package this feature is related to

stream_chat_flutter

Which platforms would this feature impact?

iOS, Android, Web, Windows, MacOS, Linux

Is your feature request related to a problem?

Currently, there is no way to display a muted channel at full opacity.

In a helpdesk system, we have an admin role overseeing all conversations.
To avoid a flood of notifications, the admin has all conversations as 'muted'.
Unfortunately, the UI for a muted conversation is non negotiable halved opacity.
Stream Channel List Tile build method:

builder: (context, isMuted) => AnimatedOpacity(
        opacity: isMuted ? 0.5 : 1,

Describe the solution that you'd like

Allow the user to override opacity by adding opacity to the constructor and copyWith of Stream Channel List Tile, with the existing logic as fallback.

This hands back a bit more conditional/contextual control to the widget displaying the list.

Describe alternatives that you have considered

Adding a field to StreamChannelPreviewThemeData, like mutedChannelListItemOpacity or muteOpacity, for widgets with conditional opacity on muted state to adhere to.

This would help with consistent/global styling, but this does seem like a change that depends on context.

Additional context

Thank you kindly for your work on making Stream available on Flutter :)

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