net/nanocoap: write struct-based block1 option

[kb2ma]

Contribution description

Adds a function to write a block1 option using the struct-based nanocoap API.

Testing procedure

See the gcoap-block server app. It is equivalent to the nanocoap_server example, and includes an /sha256 resource. The handler writes a block1 option in the responses. Note that the RIOTBASE variable in Makefile does not use the RIOT submodule.

For a more complete test, see pytest-based block1_test.

Issues/PRs references

Part of the implementation of #9309.

[kb2ma]

This PR unfortunately was premature. My original intention was to allow writing a block1 option in control usage using the struct-based API, similar to coap_put_block1_ok(). It seemed an easy thing to do, and it was.

However, my larger goal is to complete the block implementation, which means addition of the client side for block1 and block2. I just started to look at this after I posted this PR. In this case, I think that the function coap_opt_add_block1() added in this PR instead will be used to write a message with the content, which uses a coap_block_slicer_t.

So, closing for now. My next step is to open an issue to describe the function names for the complete block implementation. Then I can revisit/rework this PR in that larger context.