[java] add desiredProperties CRUD support

[andrew-kratik]

In order to use the ditto-client to manage desired feature properties, the operations have to be executed via creating ditto-protocol messages manually in the client link In communication pattern when properties are set by device only and desiredProperties are set by server - special CRUD operations for desired feature properties are missing in the client.

Would be great to add desiredProperties support for java client to keep it in sync with ditto-protocol.

[thjaeckle]

That is indeed a valid request - seems that this was not done when adding the desiredProperties to Ditto itself. In addition to CRUD operations, also registering for desiredProperties changes would be needed to cover it all.

I don't think that the Ditto team will work on that in the near future (as you said, it is possible to manually send the CRUD operations with the client, just not as convenient). If this is an important addition for you, we will gladly accept a contribution adding this to the Ditto Java client.