Log Message:
grpc: updated to 1.28.1

Release v1.28.1
This release is a patch specifically for the Python bindings to address 22546.

Python
Backport 22549 to 1.28 (Only load simple stubs code on 3.6+ interpreters).

Release v1.28.0
This release contains refinements, improvements, and bug fixes, with highlights \ 
listed below.

Core
feat: add x-goog-user-project header from quota_project_id field.
Reintroduce 21527 (boringssl submodule unification - take two).
Reintroduce 21527 (boringssl submodule unification).
Treat an empty http_proxy mean "Don't use proxy" and skip parsing it.

C++
This release adds an experimental client side implementation of xDSv2 APIs. This \ 
allows a gRPC client written in C++ to receive configuration from an xDSv2 API \ 
compatible server and use that configuration to load balance RPCs. In this \ 
release, only the virtual host matching and cluster route action is supported. \ 
More features will be added in future.
Cherrypick 22147: xds: send no-overprovisioning client capability and populate \ 
user_agent fields.
Fix regression in MSVC runtime flag.
Unify boringssl submodules and use non-developer boringssl cmake build.

Objective-C
Fix ObjC memory consumption increase as app runs.

Python
Backport Simple Stubs Codegen to 1.28.
[Aio] Generate gRPC AsyncIO API Reference.
Experimental: Implement Top-Level Invocation Functions Not Requiring an Explicit \ 
Channel.
[Aio] Implement health checking servicer in AsyncIO.
[Aio] Prohibit mixing two styles of API on client side.
[Aio] Add channel_ready helper function.
[Aio] Support compression for both client and server.
[Aio] Close ongoing calls when the channel is closed.
[Aio] Fix the server credentials & improve socket implementation.
[Aio] Support wait-for-ready mechanism.
[Aio] Fix the windows build failure.
Stop building manylinux1 images.
[Aio] Move status and initial metadata handling to Cython.

Ruby
Backport to 1.28.x: add ruby 2.7 to mac binary pkgs.
Add ruby-2.7 to build process of binary gems for Windows and Linux.