Add GitHub bot to check Apache license headers, on pull-requests

[NimJay]

• We should enable the License Header Lint GCF GitHub bot on this repo.
• The bot runs a GitHub check called header-check that warns us about invalid/missing (Apache 2.0, etc.) license headers.

Background

• When a new file is added to this repo, we sometimes forget to add the Apache 2.0 license header.
• According to Google-internal docs:

It is necessary to apply a Google copyright notice to any source file that is published or shared outside of Google

• and ...

All published source files should have some form of license header with at least one copyright notice.