Support community maintained Github action test

[gyuho]

We and including others have wanted to add custom CI for a long time:

• Support for Raspbian on RPi4 or armv7l? #12852
• Set up CI for multi-arch such as s390x #11163

Self-hosted GitHub action runner provides a consistent way of hosting various testing workers. So, we are proposing to add a second-tier of CIs that run on top of GitHub action + self-hosted runner. This will enable us to increase our testing coverage across various providers, without impacting our current code review process (we can add as non-blocking pre-submit tests).

We've already experimented this idea with an AWS-hosted EC2 instance to test etcd with ARM-based machine. And happy to contribute, so other providers can refer and set up their own test workers.

/cc @ptabor @jpbetz @hexfusion @PatrickXYS @retroflexer

[stale]

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