gcoap: improve block-wise transfer API. #16715
Description
Description
After working a bit with block-wise transfer in both gcoap and libcoap, I kind of hate how block-wise transfer is handled in gcoap. A lot of work is already taken of once shoulder by using the slicer (which is also non-obvious, because there are no examples for that in the repo), but still there is a lot of in-depth knowledge about RFC 7959 required to enable you to exchange large data. libcoap solves quiet nicely (but still far from perfect), by having _large_ versions of functions that could require block-wise transfer that hide most of the mechanics of block-wise transfer from the user. A similar user-friendly approach would be desirable.