net/nanocoap: get option bytes

[kb2ma]

Provides the ability to retrieve the value for an option as an opaque array of the bytes for the option. It is useful for an option with an opaque (byte array) data type. It also is useful to access any option as a byte array. For example, it can be convenient to replace the more attribute for a block option after it has been written to the packet.

In future PRs we plan to refactor the implementation of the internal coap_get_option_uint() to use coap_opt_get_opaque(), rename it to coap_opt_get_uint() and make it public. We also plan to refactor coap_get_blockopt() to use coap_opt_get_opaque().

Testing procedure

Added to the tests-nanocoap unit tests.

Issues/PRs references

Depends on #12074. Partially replaces #11437.

[smlng]

please rebase, #12074 is merged

[kb2ma]

Rebased and engaged CI.

[smlng]

looks good to me, nice addition

[chrysn]

As a replacement for the coap_opt_get_next construction from a pointer, I suggest we add an option iteration initializer that'd work from coap_find_option. Thus, single options would be accessed using this coap_opt_get_opaque(pkt, optnum, ptr), but repeated ones using coap_opt_seek(pkt, optnum, &optpos), and can then be read single-file by coap_opt_get_next(pkt, &optpos, &value, false).

[kb2ma]

@chrysn and I discussed on PR 12074. No changes required at the moment.