net/gcoap: Make references to coap_resource_t const in gcoap

    A CoAP resource is a primary object between the application
    and CoAP library.  The Library needs the paths, methods,
    and handlers from it, so that it can call the right handler.
    However, it never needs to change any of them.

    The application also needs the resources.  The application
    may want to declare the resources as const, since it may
    want to store them in flash.