feat: spread tests and centralized justfile

[lucabello]

This PR brings the rock up to speed with our newest practice of a centralized justfile, and changes our tests to use rockcraft test (and thus spread).

I'm adding spread tests to the latest folder only; feel free to copy-paste that to other versions if you want, but please do so in a follow-up PR!

Context

More information can be found at:

• rocks blueprints' README
• feat: add blueprints observability#422

Per usual, you can also find the available commands by simply running just.

FAQ

• Can we remove the .extension file? The .extension file is a rockcraft init --profile=test artifact; unfortunately we can't remove it or avoid to commit it
• Does this model allow for version-specific tests? Yes, each version has its own set of spread tests
• Can we symlink spread.yaml instead of copying it in just test? Unfortunately not, as rockcraft test (which internally calls rockcraft pack first) mounts the rock folder to a LXD container for packing, which breaks all symlinks

Testing Instructions

just test will run the tests for the latest rock version.

Upgrade Notes

This PR won't trigger changes to any rockcraft.yaml file, which means:

• no rocks will be changed
• no tests will be triggered on this run
• no PR will be opened to OCI Factory
• the latest rock version will still be pushed to GHCR under the :dev tag