Check for major charm library updates

[lucabello]

What needs to get done

charmcraft fetch-libs should offer a way to check for major charm library updates, or even pull them in.

Currently, charmcraft fetch-lib updates charm libraries to the latest LIBPATCH, which provides a way to check for minor upgrades, but there's no way to detect when there's an update with a higher LIBAPI.

Why it needs to get done

We need a way to detect whether a charm library has a new major update or not. The idea is for a CI workflow to open a GitHub issue in a repository when that happens, so it can be tracked and prioritized properly.