mgr/dashboard: add OSD recovery speed form

mgr/dashboard: add OSD recovery speed form

Add a form to set the OSD recovery speed. There are three predefined
priorities available: 'Low', 'Default' and 'High'.
By setting the priority the config options 'osd_max_backfills',
'osd_recovery_max_active', 'osd_recovery_max_single_start',
'osd_recovery_sleep' will be updated with predefined values.
It's also possible to set custom values.

Fixes: https://tracker.ceph.com/issues/35540
Signed-off-by: Tatjana Dehler <tdehler@suse.com>