samples: can: Add example code for CAN driver

This adds example and testing code for CAN driver.
Tested on stm32f072b disco.
Examples are given for:
- can_configure
- can_attach_isr
- can_attach_msgq
- can_send

Signed-off-by: Alexander Wachter <alexander.wachter@student.tugraz.at>
7 files changed