[#1582] CoAP endpoints in Ditto gateway

[thjaeckle]

Initial draft of CoAP support in Ditto gateway:

• provides (currently unsecure, plain UDP) CoAP endpoint
• providing equivalent CoAP resources as the HTTP resources:
  • /things, /policies, /whoami
    • and all of the existing subresources of the above, see HTTP API
  • supporting verbs: GET, PUT, POST, DELETE, PATCH, IPATCH
  • managed to re-use the complete code for the Ditto HTTP routes without the need to do an additional network "hop" to HTTP
• providing "observe" functionality for watching changes of resources:
  • managed to use existing "Server Sent Event" functionality for getting change notifications
  • including all of the features of SSE, field selectors, extra fields, etc.
  • managed to "prolong" existing observe-sessions when client observes again after "max-age" with the same "CoAP token"
  • "unobserve" cancels the backend subscription for changes

Open TODOs:

• configuration of all Californium CoAP related settings (e.g. max payload sizes, timeouts, etc.)
• coaps - CoAP via DTLS and different authentication types (PSK, Certificates)
• ensure receiving Ditto "messages" (from inbox/outbox) works via "CoAP observe"
• cancel observe streaming sessions once 1 or 2 "observe" prolongements from a client were missed
• ensure timeout handling works, e.g. if Ditto takes too long for a request to answer
• check/fix threading in the added DittoCoapResourceFacade - is single threaded enough? probably not
• register the DittoCoapServer in CoordinatedShutdown
• resolving open TODOs in code
• provide unit tests
• provide system-tests testing with a CoAP client
• add documentation

Eventually resolves: #1582