feat: create toolchain type for py_proto_library (bazel-contrib#1416)

This is to eventually allow py_proto_library to use toolchain resolution
for getting
the Python-specific protobuf information necessary to create protos.

Design doc:
https://docs.google.com/document/d/1CE6wJHNfKbUPBr7-mmk_0Yo3a4TaqcTPE0OWNuQkhPs/edit#heading=h.5mcn15i0e1ch

Author: comius

python/proto/BUILD.bazel

@@ -0,0 +1,18 @@
+# Copyright 2023 The Bazel Authors. All rights reserved.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#    http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+package(default_visibility = ["//visibility:public"])
+
+# Toolchain type provided by proto_lang_toolchain rule and used by py_proto_library
+toolchain_type(name = "toolchain_type")