Upport kokoro/release/python from 3.6.x branch

Down-integrate from google3.

Add experimental collect_all_artifacts script

Update README.md with gradle instructions for java

* Fix conformance running nothing issue

This change adds a source dependency of the test suite implementaion
class in the main function. For generality reason, the main function is
moved to the file of the test suite implemetation.  New test suite
implementation will need to implement the main function.
In order to make it easy for test suite implementation to implement the
main function, this change also refactor out the common code out of the
main function.

* Fix typo

Added conformance test impl file to CMake.

Fixes JSON Loading of Any messages with Unknown Fields  in C++

Added cmake build files for the conformance test runner.

Add contributing guidelines.

* Fix well known type class not inheriting Message

Previously, well known types only implement Message interface but not inhert it.
Fix the problem by calling zend_do_inheritance

* Add back commented tests

* Add missing reference to conformanc_test_impl.cc in Makefile

* Update conformance failure list

Credit goes to @mercret for the fix.

Use ./tests.sh cpp to run it

update version

Request new option extension (and update URLs for existing projects)

Update options.md | fix website url for C# port

Update my projects' (DCCL / Goby) URLs.
Propose new extension for CGSN Mooring project.

I want to use the conformance tests from upb, which is switching
to CMake.

Update options.md | fix website url for C# port of protocol buffers.. From `http://github.com/jskeet/dotnet-protobufs` to `https://github.com/jskeet/protobuf-csharp-port`

Revert changed links in Dockerfiles.

These changes require building new docker images and without new images
our kokoro tests are all failing.

Replace repo links.

Related to #658 and #5007.

Fix TryCreateParentDirectory on Windows