[CoAP rework] coap_req: introduce new helper module

[mniestroj]

Introduce new helper module, which will handle most low-level CoAP specific tasks like:

• contructing CoAP packets for the most common use cases,
• keeping track of pending (unacked) CoAP packets,
• retransmitting packets with exponential backoff,
• requesting wakeup from poll() system call, when new packet is to be transmitted or there is a retransmission of pending packet needed,
• handling block-wise transfers (requesting next blocks of data, if response contains information about block number),
• providing thread-safe APIs.

Add golioth_prepare_poll() function will allow to get file descriptor and next timeout value, so those values can be passed to subsequent poll() system call. This is used by system_client.c implementation, but the idea is to split it into application specific custom "system client" implementation.

TODO:

• [x] DFU samples fails to build for ESP32 (DRAM overflow)
• [x] to be tested with nRF91 modem sockets, where wakeup might not work (due to no suport of waking up from poll())
• [x] wrap offloaded sockets to allow poll() be interrupted (part of #298)

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 6ced72c):

https://golioth-zephyr-sdk-doxygen-dev--pr270-coap-req-5mupsuoz.web.app

_{(expires Fri, 14 Oct 2022 23:25:36 GMT)}

_{🔥 via Firebase Hosting GitHub Action 🌎}

[ncmiller]

to be tested with nRF91 modem sockets, where wakeup might not work (due to no suport of waking up from poll())

Is this still a TODO?

[mniestroj]

to be tested with nRF91 modem sockets, where wakeup might not work (due to no suport of waking up from poll())

Is this still a TODO?

Tested. Unfortunately poll() does not support being interrupted with offloaded sockets (which nRF91 sockets are). Created https://golioth.atlassian.net/browse/DSDK-390 to address that.