[Python] Generate .pyi stub files when --python-typing is on. (#8312)

* [Python] Generate `.pyi` stub files when `--python-typing` is on.

To support this change, the following modifications were made:

-  added a new option to disable `numpy` helpers generation;
-  added a new flag to control the target Python version:

   `--python-version` can be one of the following:

   - `0.x.x` – compatible with any Python version;
   - `2.x.x` – compatible with Python 2;
   - `3.x.x` – compatible with Python 3.
-  added codegen utilities for Python;
-  added a note that the generated .py file is empty.

* [Python] Update Bazel build rules.

* [Python] Update Bazel build rules.

* [Python] Run buildifier on BUILD.bazel files.

---------

Co-authored-by: Derek Bailey <derekbailey@google.com>

include/codegen/BUILD.bazel

@@ -0,0 +1,24 @@
+load("@rules_cc//cc:defs.bzl", "cc_library")
+
+package(
+    default_visibility = ["//visibility:private"],
+)
+
+filegroup(
+    name = "distribution",
+    srcs = [
+        "BUILD.bazel",
+    ] + glob([
+        "*.cc",
+        "*.h",
+    ]),
+    visibility = ["//visibility:public"],
+)
+
+cc_library(
+    name = "python",
+    srcs = ["python.cc"],
+    hdrs = ["python.h"],
+    strip_include_prefix = "/include",
+    visibility = ["//src:__subpackages__"],
+)