Build & test pack images against StackStorm versions

[Stealthii]

In Stackstorm 3.8, Python v3.6 was the recommended supported environment. With 3.9dev still in the works, this has changed to Python 3.8 through 3.10, with more changes likely down the road as this version develops.

https://github.com/StackStorm/st2packs-dockerfiles provides 3 images suited for building pack images to use with the K8S setup for StackStorm, however they are built upon, and therefore only support the stable (3.8) stackstorm version at this time. We should be able to build and test StackStorm packs against multiple supported Stackstorm (and if applicable, Python) versions.

https://github.com/Stealthii/st2packs-dockerfiles/tree/feature/refactor tracks a rewrite to this image pipeline structure, utilising one image for the build that allows for pack images to be built against a given stackstorm version. There are separate purposes for this:

• Pack images should be explicitly built against the version of Stackstorm they are being mounted against. There is no guaranteee that a pack image built against StackStorm 3.8 (py36) will work with StackStorm 3.9 (py310)
• Packs can be built in a pipeline against all supported (and future) versions of StackStorm, with the ability to signal supported StackStorm versions in the Exchange eventually.

For the community, I'm interested in what requirements we want to lay down to this process, i.e. should all packs signal and be tested against StackStorm versions? should there be clearer definitions for a test framework that packs should use to ensure functionality?