bluetooth: services: nus: Add Nordic UART Service

NUS is implemented as a Bluetooth LE service, exchanging data treating
the NUS characteristics as Serial endpoints: RX characteristic to
receive, TX characteristic to send binary packets.

This implementation also enables the ability to define multiple
instances of the NUS Service, analogous to mutliple serial endpoints,
to use each one for different purposes. Unless disabled through Kconfig,
NUS instantiates a default instance, similar to what other services do,
which allows users not interested in using multiple instances, to not
worry about the inherent complexities.

Signed-off-by: Luis Ubieda <luisf@croxel.com>