Store the user's Launchpad credentials in a keyring when available

### What needs to get done

`craft-store` uses `keyring` as its backend and `launchpadlib` has a way to store credentials in a keyring rather than a file.

We should still have a way to fall back to a file when no keyrings are available, but we might be able to use [craft-store's FileKeyring](https://github.com/canonical/craft-store/blob/f9171f1bfedd3653cc764b91cf4a0193bc1e431a/craft_store/auth.py#L82-L143) to do that.

### Why it needs to get done

Better not to use a file if we can avoid it.

Start by reviewing how craft-application currently configures launchpadlib credentials and the linked craft-store FileKeyring implementation. Check launchpadlib's keyring storage path and define the fallback behavior when no keyring is available. Done means credentials use a keyring when available and a file fallback remains functional.

Written by the indexing model from the issue text.