Java implementation for real-time notifications

[Christian-Winter-Software-AG]

This patch provides the tools which enable users to handle real-time notifications easily from within their Java applications. The patch

• covers all types of domain model objects that support real-time notifications,
• covers all rea-time notification endpoints,
• provides proper Java representations for the involved notification objects (including conversion to/from JSON based on the Svenson library generally used for JSON handling in the Java SDK),
• can handle DELETE operations while not losing type information for CREATE/UPDATE notifications,
• includes some unit tests,
• is easy to use, and
• has appropriate documentation.

See also internal ticket IOT-19410.