wifi-types Client_State needs state for clients not in active 802.11 state machine.

[jsnyder81]

openconfig-wifi-types Client_State does not have the capability to show a client that is still present in the state table, but is currently inactive. This inactivity could be caused by it roaming or disconnecting, and the AP in question has not purged the details from the client database for performance reasons.

We need to introduce a state that indicates the client is still present in the AP output, but mapped to a state that can be filtered upon.