downstream projects tests with GHA

[azmeuk]

Checks if the main branch of the various downstream projects (scim2-client, scim2-server, scim2-cli) are still compatible with scim2-models.

Supersedes #56

[azmeuk]

@ccoors after giving some thoughts, I find this way less intrusive than submodules. I will merge this instead of #56. Plus it has the benefit of always testing the latest commit from the downstream projects, so we won't need to update the submodules every time.
I made you co-author of the commit.