Support for specifying WAL/DB device parameters for MicroCeph OSDs from the MicroCloud init (Manual or Preseed)

Issue:

MicroCeph improves performance for spinning disks (HDDs) by using a separate SSD partition for the write-ahead log (WAL) and the BlueStore database. This is explained in the [`microceph disk add`](https://canonical-microceph.readthedocs-hosted.com/latest/reference/commands/disk/#add).
However, the current MicroCloud init flow (both manual and preseed file) does not support specifying the additional configuration parameters. This prevents us from creating Ceph clusters that use a separate WAL/DB partitions using the `init` flow.

Workaround used for the current use case:

Currently we are initializing the MicroCloud using the preseed file with the minimum number of OSDs. Then once the MicroCloud cluster is ready, we are adding the disks using `microceph disk add --wipe --db-device --db-wipe` command.