ceph-volume: add python hints to util.prepare.create_id()

ceph-volume: add python hints to util.prepare.create_id()

This commit introduces type annotations to the `create_id` function in `ceph_volume.util.prepare`.
The parameters and return value are now typed as follows:
  - `fsid` is a `str`.
  - `json_secrets` is a `str`.
  - `osd_id` is an optional `str` (`Optional[str]`).
  - The function returns a `str`.

Signed-off-by: Guillaume Abrioux <gabrioux@ibm.com>