Bluetooth: GATT: Add function to check if attribute has been subscribed

Add public API function in GATT to check if a specific connection has
subscribed to the given attribute.
Without this function the application has to keep track of which
connections has subscribed using the callbacks from BT_GATT_CCC_MANAGED
since the cfg_changed callback of the CCC doesn't carry connection
context.
The other alternative is for the application to parse the information
in the struct _bt_gatt_ccc object. Although this object has structure
information available to the application the structure is marked as
internal, so the application shouldn't rely on this definition.

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>