add support for presentation format descrictor when defining a characteristic

Refer to the user request: https://developer.mbed.org/questions/61200/Setting-BLE-Characteristic-Units/

We need to support the presentation format descriptor when defining characteristics.

Read the linked user request first, then locate the API used for defining Bluetooth characteristics. Confirm how a presentation format descriptor should be represented and determine what existing characteristic-definition behavior must change; done means characteristics can be defined with that descriptor.

Written by the indexing model from the issue text.