hostapd: ubus: add BSS transtiton request method

hostapd: ubus: add BSS transtiton request method

The existing wnm_disassoc_imminent ubus method only supports issuing a
bss transition request with the disassoc imminent flag set.
For use-cases, where the client is requested to roam to another BSS
without a pending disassoc, this existing method is not suitable.

Add a new bss_transition_request ubus method, which provides a more
universal way to dispatch a transition request. It takes the following
arguments:

Required:
addr: String - MAC-address of the STA to send the request to (colon-seperated)

Optional:
abridged - Bool - Indicates if the abridged flag is set
disassociation_imminent: Bool - Whether or not the disassoc_imminent
                         flag is set
disassociation_timer: I32 - number of TBTTs after which the client will
                      be disassociated
validity_period: I32 - number of TBTTs after which the beacon
                 candidate list (if included) will be invalid
neighbors: blob-array - Array of strings containing neighbor reports as
           hex-string

Signed-off-by: David Bauer <mail@david-bauer.net>