mgr/dashboard: replace string version with class

mgr/dashboard: replace string version with class

* APIVersion:
  * Moved to a separate file
  * Added doctests
  * Added sentinel values:
    * DEFAULT = 1.0
    * EXPERIMENTAL = 0.1
    * NONE = 0.0
  * Added to_mime_type() helper method
* Controllers.__init__:
  * Added type hints
  * Replaced string versions with APIVersions
* Feedback controller:
  * Replaced with EXPERIMENTAL (probably it should be NONE)

Fixes: https://tracker.ceph.com/issues/52480
Signed-off-by: Ernesto Puerta <epuertat@redhat.com>