[internal] Continuous Integration on GitHub Actions

[franky47]

This issue tracks progress of moving continuous integration from TravisCI to GitHub Actions.

CI steps:

• [x] Build the library code via CMake
• [x] Build and run unit tests
• [x] Generate code coverage and upload to Coveralls
• [x] Use PlatformIO to build all examples with a variety of officially supported Arduino (and variants) boards (eg: Teensy, ESP32).
• [x] Reduce checks verbosity for examples matrix
• [ ] Deduplicate checks for PRs (push + PR sync creates redundancy)

CI will run for each pushed set of commits on any branch, and on PRs.

A passing CI should be required on the master branch.