Make a taskKey applicationDescriptor available for external use

[Lockdain]

Is your feature request related to a problem? Please describe.
Application Descriptor is very useful since it contains almost any pipeline-related info.
Currently the taskKey applicationDescriptor at CloudflowKeys.scala marked as private and can't be utilized by user in any way.

We've observed a use cases when it is needed to exchange a pipeline's topology (I mean basically its structure) with other systems, e. g. frontend. It would be much more simpler to create any topology integration structures based on App Descriptor object (by creating custom sbt task at the user side).

Is your feature request related to a specific runtime of cloudflow or applicable for all runtimes?
Local runtime basically.

Describe the solution you'd like
Make Application Descriptor public or create its replica, which will be available for users.