Bluetooth: Shell: Add support for EAD

Add new commands to manage the Encrypted Advertising Data feature.

Overview of those new commands:

- `bt encrypted-ad set-keys`: set key materials (session key and
  initialisation vector) used for encrypted and decryption of EAD;
- `bt encrypted-ad add-ad`: store a given advertising data structure;
- `bt encrypted-ad add-ead`: encrypt the given advertising data
  structres and store the generated AD structure;
- `bt encrypted-ad commit-ad`: set the AD of the selected advertiser
  with the stored AD;
- `bt encrypted-ad clear-ad`: remove all stored AD;
- `bt encrypted-ad decrypt-scan`: decrypt data using the previously set
  key materials when receiving AD with type `0x31`.

The documentation of the Bluetooth Shell has been updated to include
those new commands.

Signed-off-by: Théo Battrel <theo.battrel@nordicsemi.no>