chore: refactor most theme related classes into a /theme folder

stream_chat_theme.dart remains in /src because I feel it will cause too much of a mess if moved.

packages/stream_chat_flutter/lib/src/theme/avatar_theme.dart

@@ -0,0 +1,44 @@
+import 'package:flutter/material.dart';
+
+/// Theme for avatar
+class AvatarTheme {
+  /// Constructor for creating [AvatarTheme]
+  AvatarTheme({
+    BoxConstraints? constraints,
+    BorderRadius? borderRadius,
+  })  : _constraints = constraints,
+        _borderRadius = borderRadius;
+
+  final BoxConstraints? _constraints;
+  final BorderRadius? _borderRadius;
+
+  /// Get constraints for avatar
+  BoxConstraints get constraints =>
+      _constraints ??
+          const BoxConstraints.tightFor(
+            height: 32,
+            width: 32,
+          );
+
+  /// Get border radius
+  BorderRadius get borderRadius => _borderRadius ?? BorderRadius.circular(20);
+
+  /// Copy with another theme
+  AvatarTheme copyWith({
+    BoxConstraints? constraints,
+    BorderRadius? borderRadius,
+  }) =>
+      AvatarTheme(
+        constraints: constraints ?? _constraints,
+        borderRadius: borderRadius ?? _borderRadius,
+      );
+
+  /// Merge with another AvatarTheme
+  AvatarTheme merge(AvatarTheme? other) {
+    if (other == null) return this;
+    return copyWith(
+      constraints: other._constraints,
+      borderRadius: other._borderRadius,
+    );
+  }
+}