sd: MTU exchange

[rdnt]

This PR adds the capability to perform an MTU Exchange procedure from the central (gatt client - usually), and the ability to configure an ATT_MTU up to 247 bytes (allowing 244 bytes of application data).

• The nrf Adapter interface has been adapted to include a Configure(cfg Config) error method that allows setting some initialization options for the SoftDevice before it is enabled.
• An ExchangeMTU(mtu uint16) (uint16, error) function was added to the Device interface, that can be called from the gatt client to initiate an MTU exchange procedure.
• The bluetooth adapter can still operate without these two function calls, in which case it will use the default configuration {EventLength=2, AttMtu=23}.

I have tested this implementation on two nicenano_v2 boards.
In my tests, I have found updating the event length can allow for increased bandwidth, so I also implemented a way to configure it.
I have made some experiments with adding Data Length Extension as well, but I reckon some initial feedback would be welcome before I delve into starting a proper implementation of DLE.

Do let me know if you would like to have a working example (maybe also in the source code?) with a 247 MTU exchange + throughput test of sorts.

Lastly, I can only test on s140v7, so I can't really verify that the peripheral-only variant (s113v7) works as expected. If someone else could lend a hand I would appreciate that.

Refs:

• Using ATT_MTU sizes that are multiples of 23 bytes is ideal

Relevant: #200 (comment), #142 (comment)

[rdnt]

Heya folks! I'm marking this as ready-for-review, hoping for an initial review pass.
I am not very familiar with BLE, so any feedback, positive or negative, is welcome.

I reckon I saw somewhere (edit: #142 (comment)) that such a change requires implementation for two different platforms. I am not sure if I can commit to implementing this also for e.g. Linux, but if required I will see what I can do. Otherwise this PR can be left open as a working POC until someone contributes a relevant implementation for some other platform.