[CoAP rework] Update DFU

[mniestroj]

Rework FW module on top of 'coap_req' module. This gives several advantages:

• Packet retransmission both with desired firmware request and with firmware download.
• Protocol agnostic application APIs, as there is no direct use of CoAP in the APIs.
• Simplified application APIs, as there is no need for manual handling of 'coap_reply', but instead there is just a single callback type for both "desired firmware information" and "firmware content".
• Significant code deduplication, as blockwise CoAP GET is handled mostly by 'coap_req' module.

Update samples/dfu/, so that it uses updated APIs.

[github-actions[bot]]

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