* chore: Add ImageHeaderTheme and ImageHeaderThemeData to stream_chat_theme.dart
This commit adds the following classes to `stream_chat_theme.dart`:
1. `ImageHeaderTheme`, an `InheritedTheme` widget that can wrap other widgets and provide that sub-tree with an `ImageHeaderThemeData`
2. `ImageHeaderThemeData`, a class that allows for the customization of `ImageHeader` widgets
* Make ImageHeaderThemeData constructor const
* test: initial ImageHeaderThemeData tests
Tests the copyWith, ==, and hashcode functions
* test: add tests for default ImageHeaderThemeData values
Tests the default values of ImageHeaderThemeData that gets built in a StreamChatThemeData against control values.
* Update test name and add comments describing control
* fix: use ImageHeaderTheme.of(context) instead of StreamChatTheme.of(context)
This ensures that ImageHeader will always look for the closest ImageHeaderThemeData in the widget tree.
* test: add dark theme default values test
* run dartfmt on test file
* test: lerping between ImageHeaderThemeData
Added tests that lerp from one ImageHeaderThemeData to another (light to dark and dark to light)
* test: merging ImageHeaderThemeData
Added tests that merge light and dark ImageHeaderThemeData together (light + dark = dark, dark + light = light)
* test: add a half-lerp test
Added a test that lerps halway between light and dark ImageHeaderThemeData
* chore: Add ImageFooterTheme and ImageFooterThemeData to stream_chat_theme.dart
This commit adds the following classes to `stream_chat_theme.dart`:
1. `ImageFooterTheme`, an `InheritedTheme` widget that can wrap other widgets and provide that sub-tree with an `ImageFooterThemeData`
2. `ImageFooterThemeData`, a class that allows for the customization of `ImageFooter` widget
* chore: apply ImageFooterThemeData to image_footer.dart
* fix: make ImageFooterThemeData constructor const
* test: add tests for ImageFooterThemeData
* add a missing trailing comma
* chore: rename ImageHeader & ImageFooter to GalleryHeader and GalleryFooter
The respective theme classes and tests have also been renamed
* Run dartfmt on test
* test: fix failing tests due to renaming
