Bluetooth: add destroy callback to indication

Adds a `destroy` callback to the `struct bt_gatt_indicate_params` which
is used to signify to the application that the indication operation has
completed and the struct instance can be freed/destroyed.

This is required as the number of indication value callbacks that will
be triggered is not known by the caller when the `conn` parameter is
`NULL`.

Tracking when this callback should be run is mananged by a private
reference counter inside the struct.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>