Add artifacts_upload step.

This adds the implementation of the artifacts_upload.py step along with
unit tests. Adds the postbuild.py main function. Adds a new step to
yaml configuration file calling postbuild.py.

Tested: I ran unit tests with pytest to test for function behaviors
of git_metadata step. All test cases passed. I also ran an end-to-end
test with the sstate-cache, build was successful.

Signed-off-by: Simon-Lii <thesimonli@google.com>
Change-Id: I8d2b85bb3a07176f334f9811d0d5dd519bfd8814