extend DiscoveredCharacteristic to allow serialization and reconstruction from persistent storage

it may be useful at times to construct DiscoveredCharacteristisc (and services) from persistent (cached) data. This will allow us to avoid service-discovery for every connection. BLE_API needs to be extended for this.

Start by mapping the BLE_API surface for DiscoveredCharacteristic and services, then identify how persistent or cached data is represented. Define the serialization and reconstruction contract, including how reconstructed objects avoid service discovery, and verify that connections can use cached data correctly.

Written by the indexing model from the issue text.