Descriptors showing on Android but not on iOS

Hi!

First, a disclaimer: I am not 100% sure this is an issue with bumble.

I have the following code (based on [run_notifier.py](https://github.com/google/bumble/blob/45ca0ef071425f3c11cb57dc2ae95c9a0f2df483/examples/run_notifier.py)), which creates a device with a service that has a characteristic that has a descriptor. These attributes (service, characteristic and descriptor) have custom UUIDs.

ble_device.py:
```
import asyncio
import os
import logging
import json

from bumble.device import Device, Connection
from bumble.transport import open_transport_or_link
from bumble.gatt import Service, Characteristic, Descriptor
from bumble.att import Attribute
from bumble.core import AdvertisingData

# -----------------------------------------------------------------------------
class Listener(Device.Listener, Connection.Listener):
def __init__(self, device):
self.device = device

def on_connection(self, connection):
print(f'=== Connected to {connection}')
connection.listener = self

def on_disconnection(self, reason):
print(f'### Disconnected, reason={reason}')

def on_characteristic_subscription(
self, connection, characteristic, notify_enabled, indicate_enabled
):
print(
f'$$$ Characteristic subscription for handle {characteristic.handle} '
f'from {connection}: '
f'notify {"enabled" if notify_enabled else "disabled"}, '
f'indicate {"enabled" if indicate_enabled else "disabled"}'
)

# -----------------------------------------------------------------------------
async def main():

print('<<< connecting to HCI...')
# hardcode the device config and the serial port

async with await open_transport_or_link('serial:/dev/tty.usbmodem1101') as (hci_source, hci_sink):
print('<<< connected')

# Create a device to manage the host
device = Device.from_config_file_with_hci('brc1k/device1.json', hci_source, hci_sink)
device.listener = Listener(device)

descriptor = Descriptor("de42bd76-cfd2-11ed-afa1-0242ac120002",
Attribute.READABLE | Attribute.WRITEABLE,
)
characteristic = Characteristic(
"de42bbbe-cfd2-11ed-afa1-0242ac120002",
Characteristic.READ | Characteristic.NOTIFY,
Characteristic.READABLE | Characteristic.WRITEABLE,
bytes(json.dumps("no value here"), encoding='utf8'),
descriptors=[descriptor]
)

service = Service("de42b830-cfd2-11ed-afa1-0242ac120002",
[characteristic]
)

device.add_services([service])
device.advertising_data = bytes(
AdvertisingData(
[
(
AdvertisingData.COMPLETE_LOCAL_NAME,
bytes('Bumble', 'utf-8'),
),
]
)
)

# Get things going
await device.power_on()
await device.start_advertising(auto_restart=True)

while True:
await asyncio.sleep(3.0)

# -----------------------------------------------------------------------------
logging.basicConfig(level=os.environ.get('BUMBLE_LOGLEVEL', 'DEBUG').upper())
asyncio.run(main())
```

device1.json:
```
{
"class_of_device": 2360322,
"keystore": "JsonKeyStore",
"advertising_interval": 100
}
```

On Android I can see this descriptor that I added, but on iOS it is not there. (In both cases, the [CCCD](https://github.com/google/bumble/blob/45ca0ef071425f3c11cb57dc2ae95c9a0f2df483/bumble/gatt.py#L118) descriptor is visible.)

Screenshot from nRF Connect (Android on the left, iOS on the right):

Also, when I use one of the pre-defined UUIDs for the descriptor (e.g. [this one](https://github.com/google/bumble/blob/45ca0ef071425f3c11cb57dc2ae95c9a0f2df483/bumble/gatt.py#L116)), it is again seen on Android but not on iOS.

One more thing, this should not be an issue with nRF Connect app, since I see this discrepancy between iOS and Android also using another (custom) app.

User @coalbr reported a [similar issue](https://github.com/NordicSemiconductor/IOS-nRF-Connect/issues/116) on IOS-nRF-Connect repository.

So, my question is: does anyone have any idea why are these descriptors not showing on iOS and how to fix this? Any help would be very much appreciated!