net/gcoap: add CoAP ping request

[kb2ma]

Contribution description

Presently gcoap supports use of a piggybacked ACK response to a confirmable request. However, #13637 illustrated the need to support separate confirmable responses, as also identified in the tracking issue #7192. A prerequisite for this support is the ability to handle empty messages, which is the format for the immediate ACK response. This PR takes the first step toward empty message support by addition of handling for CoAP ping requests.

Specifically, this PR allows creation of a ping request (empty CON with code 0.00) and generation of a reset response (empty RST also with code 0.00). This PR does not include handling for the reset response because that requires empty message support for request/response correlation, which will follow in a separate PR.

Commit	Description
2d3eb1a	Validate empty message length in nanocoap, including unit test
f5a85e2	Add handling for a ping request in gcoap. Update debug msg if receive an empty ACK or RST.
6eae4de	Extend gcoap_req_init() to generate ping request if code is 0.00, including unit test
c267633	Add 'coap ping' sub-command to gcoap example

Testing procedure

• Run nanocoap and gcoap unit tests
• Set up two native terminals or boards with gcoap example. Turn on debug in gcoap.c or use Wireshark to watch exchange. Use CLI 'coap ping' command to send a ping request. Notice receiver responds with reset message, but sender doesn't recognize it. Sender times out after the expected retries.
• Also verify regular request/response handling still works with this setup.
• Review doc for gcoap_req_init()

Issues/PRs references

See above.

[leandrolanzieri]

Tests still pass and the ping packet is generated correctly. Also, normal operations work as expected, including the proxy:

• Ping:
  
• Ping response:
  
• Proxy still works:
  

[leandrolanzieri]

Changes look good and functionality works as expected. ACK.
Nice feature!