Consider making certain snaps optional on ubuntu-classic installs

We're evaluating the use of Ubuntu with TPM+FDE for workstation systems, but have found that certain snaps like `firefox`, `snap-store`, and `firmware-updater` cannot be removed because they are part of the model assertion.

In particular, the `firmware-updater` snap doesn't fully function on TPM+FDE systems anyway: https://bugs.launchpad.net/ubuntu/+source/fwupd/+bug/2065874

Attempting to remove it gives the following error:
```
error: cannot remove "firmware-updater": snap "firmware-updater" is not removable: snap is required
```

Arguably these apps are not essential to the running of the system, unlike `pc-kernel`. Do they need to be enforced always?