gcoap: move tl_type to coap_request_ctx_t

[benpicco]

Contribution description

We don't have to carry this information inside every coap_pkt_t, it's enough to provide it via the request context information.

Testing procedure

examples/gcoap_dtls still works

> coap get fe80::7837:fcff:fe7d:1aaf 5684 /.well-known/core
gcoap_cli: sending msg ID 7277, 23 bytes
gcoap: response Success, code 2.05, 46 bytes
</cli/stats>;ct=0;rt="count";obs,</riot/board>

Issues/PRs references