Update Content-Format list

[chrysn]

The ContentFormat enum currently only supports the six content formats defined in RFC7252.

Please update that list to the latest IANA published state or allow extension (eg. Other(u16)). No further support by the CoAP library beyond extending the list is required; applications need to then parse their respective formats themselves anyway, but currently can't set a content-format not in the list from 2014.

My particular application will need the application/cbor and application/senml+cbor types, but it probably makes sense to batch-import all the currently registered types.