Add Support for Github Actions CI

[ParthPratim]

Signed-off-by: Parth Pratim Chatterjee parth27official@gmail.com

Adds support for Github Actions CI/Test Workflow

Key Features :

1. Build Time :
   1. Non-Cached : 36-42 minutes
   2. Cached : 24-26 minutes
2. ns-3-dev HEAD commit based cache loading (saves around 16 mins of build time)
3. Simple list based job addition using job matrix
4. Custom non-sudo docker
5. Native target repo checkout usage
6. README badge for build status

ToDo :

1. Add a feature to support dynamic bake configurations through a comma based delimiter.

This PR was made as a part of Google Summer of Code 2021 for the project Direct Code Execution Modernization

[ParthPratim]

ToDo: Add delimiter based dynamic configuration for bake

[ParthPratim]

I just added the Ubuntu-20.04 docker and disabled Ubuntu-16.04.

The branch would show a failing sign as of now in GActions because the current ns-3-dce repo code isn't in sync with Ubuntu-20.04. But, post merging PRs for adding Ubuntu-20.04 support the build and tests should pass with a green.