Add GitHub CI workflows?

Since most contributions to this repo are going to come from community contributors for the foreseeable future, and nobody except Google employees can see the Kokoro build logs, it would be nice to add GitHub workflows in addition to the Kokoro CI.

Ideally, the GitHub runners would mirror the configurations of those used in the Kokoro CI (same distro, same compiler versions, cmake, etc.). However, the configuration for the Kokoro runners seems to be rather opaque. the only things I can only see from my end seems to be the CMake and gcc versions for the Ubuntu CI.

if you are allowed to, and you think that this is a good idea, can you post the configurations of the Kokoro runners? If so, I can put together workflows for the GitHub CI.