Shard 64-bit Linux languages into different Kokoro jobs

Migrate Jenkins jobs to Kokoro

Add support for Windows ARM64 build

Calling Keychecker before checking key in MessageMap

resolved issue 4032 and added a unit test

Fixes #4060

Fix file permission for python package.

`umask 0022` makes sure the created package will be accessible by all users.

Java: generate an annotation to suppress deprecation warnings

Add deprecation annotation for oneof case.

The valueOf method on enums is deprecated, but for compatibility reasons
the generated code has to keep using it. We can use an annotation to
suppress the deprecation warnings.

* Add auto-generated header to C# generated files

* regenerate C# protos

 Avoid double underscores in macro name

Added our standard license header to structurally_valid.cc and its test

 Add unknown field support for csharp

Fixes https://github.com/google/protobuf/issues/3268

Clarify default value behavior in JSON conversion.

This fixes issue #1775.

Update comments for Timestamp JSON format.

fix undefined behavior in C++03

Make //:protobuf_python have correct __init__.py.

Make it clear default value is only omitted for proto3.

Clarify that JSON parser are required to accept both UTC and other
timezone offsets.

Fixes issue 3093

With this in place, all tests should pass.

Fixes #3725.