Set OSD pool size when creating `ceph` and `cephfs` storage pools

[masnax]

Closes #14006

Adds the keys ceph.osd.pool_size and cephfs.osd_pool_size (in keeping with the other naming schemes).

By default, if no value is supplied, the pool size will be pulled from the global default pool size. It can be set to any value larger than 0.

On update, we will try to re-apply the OSD pool size value, if it has changed.