CoAP: Support falling back to polling observable properties and events

With the merge of https://github.com/w3c/wot-binding-templates/pull/353, the CoAP Binding Template received an update that removed the `cov:observe` subprotocol and instead specifies that a consumer should be prepared to fall back to polling when trying to use CoAP observe if the server's initial response does not contain an Observe option. To support this behavior, either the CoAP implementation needs to be updated or support for this more general interaction pattern needs to be added to the underlying CoAP libraries.

For events, we should support passing `subscription` data to a Thing via the CoAP FETCH method. For this, both the consumer and the server implementation needs to be adjusted.