add some basic ci configuration

[Mic92]

untested. Let's see if it builds.

[Teifun2]

Thanks for starting this! Should this be merged now to check that the basic debug build action works? or do you want to add the uploading of apks before that?

[Mic92]

You can merge this as it is. It already provides assurance that PRs don't break the build. Uploading APKs needs be done in a seperate github action, which is only triggered on releases so there is a release tag where files can be uploaded to.

[Mic92]

Here is a successful run: https://github.com/Mic92/nextcloud-cookbook-flutter/runs/910575712?check_suite_focus=true

[Teifun2]

Thanks for your help!