added coap overflow fault and ability to test it

Description

If too many messages are sent in a short period of time, this can overflow the CoAP message buffer on the mainboard. A new mainboard fault was added to partially address the issue. The fault is handled as an estop.

The fault can be cleared by enabling the robot if the buffer is no longer full.

This merge request includes a way to trigger the overflow fault for testing.

See burt/firmware/burt-firmware!399 for more info.

What to focus on

Test triggering and clearing the fault with the FaultHandlingTest program.