build: Add GitHub bot to check license headers

[NimJay]

• This fixes #258.
• Docs about configuring the bot.
• It's unreasonable to expect that we cover all possible file extensions for this repo, so (for now), let's just cover comment-supporting file extensions that appear at least 3 or more times.
• I used a command to check the frequency of each file extension. Warning: The command misses files without dots (e.g., Dockerfile, Makefile). See output below:

  30 java
  23 yaml
  15 sample
  13 md
  10 png
   9 xml
   8 properties
   7 js
   5 txt
   5 json
   4 vue
   3 yml
   2 config
   1 sh
   1 rev
   1 py
   1 pack
   1 npmrc
   1 idx
   1 ico
   1 gitkeep
   1 gitignore
   1 gif
   1 eslintignore
   1 css
   1 cmd

• ✅ = included in header-checker-lint.yml
• I've also included Dockerfile which isn't capture by the above command/output because it doesn't contain a dot.
• How to review: Notice the header-check GitHub check running against this pull-request.
• If you want to see a failing example, see this kubernetes-engine-samples pull-request.

[conventional-commit-lint-gcf[bot]]

🤖 I detect that the PR title and the commit message differ and there's only one commit. To use the PR title for the commit history, you can use Github's automerge feature with squashing, or use automerge label. Good luck human!

-- conventional-commit-lint bot https://conventionalcommits.org/