feat(ui_shared): extract common widgets to a shared library (#10620)

* feat(ui_shared): ThemedColor added

* add more widgets

* drop comments from pubspec

* more tests

* release preparations

* more API docs

* lower sdk constraint to match everything else

Author: lesnitsky

Diff for: packages/firebase_ui_auth/example/pubspec.yaml

@@ -38,6 +38,7 @@ dependencies:
   firebase_ui_oauth_twitter: ^1.0.24
 dev_dependencies:
   drive: ^1.0.0-1.0.nullsafety.1
+  firebase_ui_shared: ^1.0.0
   flutter_facebook_auth: ^4.4.1
   flutter_driver:
     sdk: flutter

Diff for: packages/firebase_ui_auth/example/pubspec_overrides.yaml

@@ -1,4 +1,4 @@
-# melos_managed_dependency_overrides: _flutterfire_internals,firebase_auth,firebase_auth_platform_interface,firebase_auth_web,firebase_core,firebase_core_platform_interface,firebase_core_web,firebase_dynamic_links,firebase_dynamic_links_platform_interface,firebase_ui_auth,firebase_ui_localizations,firebase_ui_oauth,firebase_ui_oauth_apple,firebase_ui_oauth_facebook,firebase_ui_oauth_google,firebase_ui_oauth_twitter
+# melos_managed_dependency_overrides: _flutterfire_internals,firebase_auth,firebase_auth_platform_interface,firebase_auth_web,firebase_core,firebase_core_platform_interface,firebase_core_web,firebase_dynamic_links,firebase_dynamic_links_platform_interface,firebase_ui_auth,firebase_ui_localizations,firebase_ui_oauth,firebase_ui_oauth_apple,firebase_ui_oauth_facebook,firebase_ui_oauth_google,firebase_ui_oauth_twitter,firebase_ui_shared
 dependency_overrides:
   intl: ^0.18.0
   _flutterfire_internals:
@@ -33,3 +33,5 @@ dependency_overrides:
     path: ../../firebase_ui_oauth_google
   firebase_ui_oauth_twitter:
     path: ../../firebase_ui_oauth_twitter
+  firebase_ui_shared:
+    path: ../../firebase_ui_shared