Bluetooth: indication callback signature update

Update the signature of the `bt_gatt_indicate_func_t` callback type by
replacing the attr pointer with a pointer to the
`bt_gatt_indicate_params` struct that was used to start the indication.

This allows the callback to free the `bt_gatt_indicate_params` instance
if it was allocated from storage, while still allowing the
`bt_gatt_attr` value to be accessed through `params->attr`.

Allocating the `bt_gatt_indicate_params` instance from storage is
desirable as multiple indications can be queued, however each instance
must be valid until the callback is run.

Implements API update from #29357

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