Add support for new API functions for TaskApp

[patptrs]

There are two new API functions provided by the TaskApp that are not yet part of the SDK.

• Retrieving a task (https://help.d-velop.de/dev/documentation/task-app#retrieving-a-task)
• Searching for tasks (https://help.d-velop.de/dev/documentation/task-app#searching-for-tasks)

I think it would be very helpful to be able to use these APIs with the SDK.

I will provide a PR.

[patptrs]

PR in #211