Contribution description

This adds a coap_opt_add_opaque(...) function that adds binary data from a buffer as an option. The name stems from the RFC7252 name for such options.

The ETag options (ETag, If-Match) are options of that form, and having a generic way of adding options also helps porting more high-level CoAP libraries to nanocoap / Gcoap. (My current use case involves using Rust CoAP abstractions on a gcoap handled server).

A follow-up commit adds const qualifiers to input data of the function itself, its internal helper (which does not modify the data in its original location) and the public coap_put_option low-level function used by it (which does neither). The behavior of the functions, both in implementation and semantically, is already const, the declarations were just missing; adding them makes using it easier with const-heavy code.

Testing procedure

None of the examples use the method, but the gcoap example can easily be made to, by adding in _riot_board_handler before (!) coap_opt_add_format:

     gcoap_resp_init(pdu, buf, len, COAP_CODE_CONTENT);
+    coap_opt_add_opaque(pdu, 4, (uint8_t*)"etag", 4);
     coap_opt_add_format(pdu, COAP_FORMAT_TEXT);

Then, a query to the riot/board resource can be verified using Wireshark to contain a 4-byte ETag option. (Tested using the native board).

Issues/PRs references

This was not reported as a dedicated issue as it's probably small enough to handle in a PR right away.