Add GattValue and FixedGattValue traits from nrf-softdevice to bt-hci

[petekubiak]

These traits will be critical for implementing the softdevice-like interface we're developing in TrouBLE (link: embassy-rs/trouble#136), and I imagine would be useful more broadly as well.

[alexmoon]

I think these are a host concern rather than an HCI concern, so should probably be in trouble rather than bt-hci. The HCI messages are inherently variable length, it's just the interpretation of that raw data by the host that needs to be aware of type conversions.

[jamessizeland]

Fair enough, our thinking was that the conversion traits would be generic beyond trouble, as it's just the same interface as softdevice uses.