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
• 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.

[Shabirmean]

:zap: Two deployments have been created for the Point-of-Sale application. You may access and test them at: :zap:

• MySQL DB backed deployment: 34.172.11.37
• Embedded H2 DB backed deployment: ****

[Shabirmean]

:zap: Two deployments have been created for the Point-of-Sale application. You may access and test them at: :zap:

• MySQL DB backed deployment: 34.172.11.37
• Embedded H2 DB backed deployment: 34.71.69.26