feat(cli): Mix in CloudAuthDatabase to database classes

To ensure that multiple databases are not needed in projects using Cloud Auth, mix in Cloud Auth to users' databases if available, or use the vendored `CloudAuthDatabase` class when there is no database.

TODO:
- Move migration logic to a proper migration class
- Run build_runner if needed

Author: dnys1

apps/cli/fixtures/fixtures_test.dart

@@ -428,8 +428,6 @@ class TestRunner {
           for (final database in project!.databases.values)
             if (database.config case ast.CelestDatabaseConfig(:final hostname))
               '--env=${hostname.name}=file::memory:',
-          if (project.auth?.providers.isNotEmpty ?? false)
-            '--env=CELEST_AUTH_DATABASE_HOST=file::memory:',
           imageName,
         ],
       );
@@ -501,8 +499,6 @@ class TestRunner {
               if (database.config
                   case ast.CelestDatabaseConfig(:final hostname))
                 hostname.name: 'file::memory:',
-            if (project.auth?.providers.isNotEmpty ?? false)
-              'CELEST_AUTH_DATABASE_HOST': 'file::memory:',
           },
           environmentId: 'local',
           verbose: false,