gcoap: Provide link encoder

[chrysn]

gcoap takes a link encoder to allow listeners customization of what they write into .well-known/core. So far, this has not been exercised by the RIOT wrappers, resulting in the use of the default handler (which just emits the path). This fell short when the resource had metadata, or when multiple resources were dispatched through Rust mechanisms under any given path.

This adds two things:

• A way for a rust-wrapped SingleHandlerListener to encode its own links
• An update to the coap_handler Gcoap wrapper that utilizes the coap_handler::Reporting information to produce sensible information.

This is all falling a bit short due to the lack of block-wise transfer in Gcoap's .well-known/core, but hey, it's a start.

[chrysn]

Now fixed to use the libc:: types from the C signature, and not the signature I copied out from the error message -- because the latter resolved type aliases in a platform dependent way (char -> i8 while it could also be u8).