net/gcoap: Replace use of gcoap_finish() with coap_opt_finish()

[kb2ma]

Contribution description

#9309 and API Options introduced the struct-based Options API for nanocoap and gcoap. As part of that work, this PR transitions gcoap to use coap_opt_finish() to mark completion of writing message metadata -- header, token, and options.

As we have developed and integrated Options, it became clear that gcoap_finish(), the original mechanism for closing a message, was not flexible enough. Once the payload is written, it becomes difficult and inefficient to attempt to insert Options in front of it. This PR fixes that rookie mistake. :-/ However, to be clear, the procedure to build a message with gcoap_finish() still works.

This PR includes gcoap's internal use of coap_opt_finish(), unit tests, and the module documentation. To limit the scope for a reviewer, we plan to update the gcoap and cord examples in a follow-up PR.

Testing procedure

The items below will exercise everything that has changed.

1. Run gcoap unit tests
2. Use the gcoap example as a server, and send requests to it for /.well-known/core and some unknown path
3. Review module documentation

Issues/PRs references

Part of #9309 struct-based Options API

[kaspar030]

ACK.

Tested gcoap example & unittests on native. Code gets a little smaller (unless DEVELHELP is enabled...)