floss: Support GATT Server connection/disconnection in btclient

Support the ability to connect to and disconnect from a registered GATT
Server in btclient.

Bug: 323943663
Test: m -j && manually with btclient
Flag: Exempt, Floss-only change

Change-Id: I6a5de7a3034db7e5d0540d2bd9d288466a62f1b6