feat: add GalleryHeader/GalleryFooter theme classes (#546)

* 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

packages/stream_chat_flutter/test/src/image_footer_test.dart

@@ -37,7 +37,7 @@ void main() {
             child: WillPopScope(
               onWillPop: () async => false,
               child: Scaffold(
-                body: ImageFooter(
+                body: GalleryFooter(
                   message: Message(),
                 ),
               ),