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/lib/src/full_screen_media.dart

@@ -6,8 +6,8 @@ import 'package:chewie/chewie.dart';
 import 'package:flutter/material.dart';
 import 'package:jiffy/jiffy.dart';
 import 'package:photo_view/photo_view.dart';
-import 'package:stream_chat_flutter/src/image_footer.dart';
-import 'package:stream_chat_flutter/src/image_header.dart';
+import 'package:stream_chat_flutter/src/gallery_footer.dart';
+import 'package:stream_chat_flutter/src/gallery_header.dart';
 import 'package:stream_chat_flutter/stream_chat_flutter.dart';
 import 'package:video_player/video_player.dart';
 
@@ -180,7 +180,7 @@ class _FullScreenMediaState extends State<FullScreenMedia>
               child: Column(
                 mainAxisAlignment: MainAxisAlignment.spaceBetween,
                 children: [
-                  ImageHeader(
+                  GalleryHeader(
                     userName: widget.userName,
                     sentAt:
                         // ignore: lines_longer_than_80_chars
@@ -198,7 +198,7 @@ class _FullScreenMediaState extends State<FullScreenMedia>
                     },
                   ),
                   if (widget.message.type != 'ephemeral')
-                    ImageFooter(
+                    GalleryFooter(
                       currentPage: _currentPage,
                       totalPages: widget.mediaAttachments.length,
                       mediaAttachments: widget.mediaAttachments,