i/b/block-devices only handle complete devices not partitions

Description

The snapd service's block-devices interface currently only allows access to complete disks while individual partitions cannot be accessed. This limitation is impeding the application of MicroCeph.

In deploying MicroCeph, the access to individual partitions of a disk presents a better level of flexibility for granular management of storage, thereby lowering the barrier of entry for deployment

Expected Result

Users should be able to access (r/w) individual disk partitions using the block-devices interface of snapd service for an easier integration with MicroCeph.

Optionally for more granularity the interface could also be implemented with an attribute to control which partitions to access, however an option should be provided to allow access to all partitions.

Thank you!