Add zephyr integration

[mkschreder]

Is your feature request related to a problem? Please describe.

It would be nice to import libcoap as a zephyr module and build it for all zephyr platforms that support networking.
Describe the solution you would like
I would like to see zephyr folder with module jaml and also zephyr cmakelists.txt file

[mrdeep1]

@mkschreder Please see #1267 as a starting point for you to try out, and comments welcome.

I am not a Zephyr developer, so there may be some better ways of doing things, but was able to run a CoAP client with no issues.

[mkschreder]

Thank you. Will have to evaluate this but looks like a good starting point so far.

[mrdeep1]

Closing as zephyr support is now available with a client example.