Note : sometimes very early early-release 3805is do not have the proper cloud cert and will not work. I should imagine that would be determined by serial number when attempting to purchase.
No the standard WS-AP3805i is not supported by ExtremeCloud(TM). Those APs do not have the necessary infrastructure to connect to ExtremeCloud(TM)
You need to actually purchase the WS-AP3805i-ROW (30913) model and with it the corresponding Cloud service entitlement.