Add support for Zephyr RTOS

[seyednasermoravej]

Hello Cisco guys.
I added support for Zephyr RTOS in libSRTP library. There is a very little changes to support Zephyr RTOS. Zephyr RTOS is a widely supported by many embedded vendors.
Here is my PR in Zephyr project.
Here is needed changes, just adding a file named module in a folder name zephyr.
How can I integrate these changes into libSRTP repo?
Thank you.

[pabuhler]

@seyednasermoravej you are welcome to create a PR that adds the folder and file and we can merge it. But there is no guaranty that it continues to build/work.
You could look at adding a github action that that tries to build for RTOS but I am not sure what is supported.

[seyednasermoravej]

@pabuhler I tested my code on two different hardware, FRDM-MCXN947 and nucleo_n657x0_q.
OK, I'll look at github action to see what I can find.

[seyednasermoravej]

Hi @pabuhler.
I created a PR here. There is a copyright commented lines based on Zephyr RTOS rules. Feel free to change it to the Cisco way.