message: build: allow disabling python proto generation
  Workaround for errors like:
  ERROR: /.../BUILD:7:1: in py_library rule //.../proto_py: paths to Python packages may not contain '-'
diff: @@ -348,7 +348,8 @@ def _proto_header(proto_path):
  return None
  # Envoy proto targets should be specified with this function.
  -def envoy_proto_library(name, srcs = [], deps = [], external_deps = []):
  +def envoy_proto_library(name, srcs = [], deps = [], external_deps = [],
  + generate_python = True):
  # Ideally this would be native.{proto_library, cc_proto_library}.
  # Unfortunately, this doesn't work with http_api_protos due to the PGV
  # requirement to also use them in the non-native protobuf.bzl
  @@ -376,6 +377,8 @@ def envoy_proto_library(name, srcs = [], deps = [], external_deps = []):
  linkstatic = 1,
  visibility = ["//visibility:public"],
  )
  +
  + if generate_python:
  py_proto_library(
  name = name + "_py",
  srcs = srcs,
  