Allow access to Service Changed Characteristic

Use case: Changing a device between BLE HID mode and some other mode where HID is disabled.

It's not so clear from the documentation if this is possible. I tried to implement my own Generic Attribute Service (UUID 0x1801) with Service Changed Characteristic (UUID 0x2A05) using `adafruit_ble.services`, but this produced an error (presumably because a service with UUID 0x1801 was already created by CircuitPython earlier in the BLE lifecycle).

The Adafruit Bluetooth Arduino libraries seem to allow the user to enable the Service Changed Characteristic via `AdafruitBluefruit::configServiceChanged` (https://learn.adafruit.com/bluefruit-nrf52-feather-learning-guide/adafruitbluefruit). Is something similar possible here?

Thank you!

Start by reading the `adafruit_ble.services` documentation and how the library creates the Generic Attribute Service (UUID 0x1801) during the BLE lifecycle. The issue asks whether users can enable the Service Changed Characteristic (UUID 0x2A05), similar to AdafruitBluefruit::configServiceChanged; determine the supported behavior and document or implement an appropriate way to access it. Check the BLE service tests for coverage.

Written by the indexing model from the issue text.