add CI automation

[cyberman54]

thanks to @t-huyeng for this PR, contributing to a long due issue, see here https://github.com/cyberman54/ESP32-Paxcounter/issues/87

Some ideas for further improvements:

• improve test coverage by automatically changing certain #define settings in paxcounter.conf (those which control compiler build options : WIFICOUNTER, BLECOUNTER, BOOTMENU, USE_OTA)
• improve test coverage by automatically changing board definitions (see suggestion in issue #87)
• maybe even add a CD test by using a test server with locally connected ESP32 boards (see suggestion in issue #87)

@t-huyeng could you do some work on this?

[t-huyeng]

I will try to work on some of these things. I also moved the steps to #87.