Allow to mutate AttributeProvider

[daschl]

This changeset makes it possible to mutate the AttributeProvider, making it possible to change attribute values during the runtime of the application.

This should at least basically cover the needs for #150

[daschl]

Here is a usage example:

let mapper:&mut BleChannelMap<ControllerServiceAttrs, NoSecurity> = ble_r.l2cap().channel_mapper();
let provider: &mut ControllerServiceAttrs = mapper.attribute_provider();
provider.set_boiler_temp(&[0x1A]); // needs some work ;-)

[daschl]

@jonas-schievink thanks for the review, updated based on your comments.

[jonas-schievink]

Thanks, looks good!