multi-arch resource support

[Simon Déziel]

The LXD charm supports deploying mixed architecture clusters as well as attaching debug binary/snap for debugging purposes. Since there is only one resource "slot" available, this is complicated for architecture specific attachments.

[Stéphane Graber (stgraber)]

We had a short chat about that last week with Juju folks and some of the notes I took were:
 - This will become more and more of an issue with operator framework charms as they have a separate build of each architecture
 - There's been some work rethinking storage and distribution of those blobs, trying to make use of cloud provided object storage and the like (currently all in the DB)
 - It would in general make sense to have those be architecture-specific with a wildcard architecture value that would cause the same blob to be provided to all units (for backward compatibility and for when the resource is data instead of architecture-dependent binaries)