Bluetooth: Add a sample to demonstrate MTU update

Add a new sample that demonstrate how to exchange MTU to allow larger
packet transmission.

The sample is split in two applications. One is the Central, it will
initiate the MTU exchange. The other one is the Peripheral and will try to
send a large notification. If the MTU exchange fail or the new size is not
big enough, the Peripheral will not be able to send the notification.

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