Support Multi-App Workflows

[olitomlinson]

Describe the feature

As of 1.16 Dapr now supports Multi-App Workflows , where Workflows and Activities can be mixed and matched across many Apps.

Please support this is in the dotnet SDK

[cicoyle]

This involves exposing support in both durabletask-dotnet + the dotnet-sdk to support this feature. There are 2 pieces to multi app workflows support, support for callactivity and support for suborchestration calls. We should support both, and have an example highlighting this new pattern in an example directory.

Reference implementation examples:
java sdk callActivity PR
durabletask-java callActivity PR
callactivity durabletask-go PR
suborchestration durabletask-go PR

Ref issue I opened in the java-sdk