[github] mark tools/distrib/python/xds_protos as auto-generated (grpc#38083)

Author: markdroth

.gitattributes

@@ -20,6 +20,7 @@ src/ruby/ext/grpc/rb_grpc_imports.generated.h linguist-generated=true
 src/ruby/ext/grpc/rb_grpc_imports.generated.c linguist-generated=true
 include/grpc/module.modulemap linguist-generated=true
 test/core/security/grpc_tls_credentials_options_comparator_test.cc linguist-generated=true
+tools/distrib/python/xds_protos/** linguist-generated=true
 tools/doxygen/Doxyfile.c++ linguist-generated=true
 tools/doxygen/Doxyfile.c++.internal linguist-generated=true
 tools/doxygen/Doxyfile.core linguist-generated=true