examples: use coap_opt_finish() in gcoap based examples

[kb2ma]

Contribution description

#10892 updates gcoap documentation and internal message generation to use coap_opt_finish() rather than gcoap_finish(). This PR updates RIOT examples that use gcoap to also use coap_opt_finish(). The examples include the gcoap CLI example, cord_epsim, and cord_ep.

Testing procedure

• gcoap cli: Send requests to the CLI for /riot/stats and /riot/board. Use the CLI to send a request. Have an Observe client register for /cli/stats, and then create another request in the CLI, which will send a notification to the observer.
• cord_epsim: Register with a resource directory server. Also, send requests to the cord_epsim endpoint itself for /riot/foo and /riot/info.
• cord_ep: Use the CLI to register with a resource directory server. Also use the CLI to discover RD servers and remove the cord_ep client from the server. Also, send requests to the cord_ep endpoint itself for /sense/temp and /node/info.

FWIW, review my request_response, cord_ep, and cord_epsim automated tests for examples.

Issues/PRs references

Depends on #10892

[kaspar030]

ACK.

tested gcoap, unittests, cord_ep (against aiocoap-rd). cord_epsim didn't show up in the endpoints of aiocoap-rd, but that might be aiocoap not speaking the protocol?

[kaspar030]

@haukepetersen could you take a quick look at this?

[kaspar030]

@kb2ma this needs a rebase.

[kb2ma]

My pleasure, thanks for the review.

[haukepetersen]

Successfully tested the cord_epsim example locally, so everything is fine with this PR.

ACK and go :-)