samples: i2c: rtio: add i2c_rtio_loopback sample

Add sample which clearly and thorougly shows how to use RTIO with
I2C controllers, both synchronously and async, comparing it with
the "standard" I2C API, in an controlled environment. Both the I2C
controller and I2C target are managed by the sample, using loopback
to monitor and control the I2C transfers.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>