Workflows: Add first class support for input/output data references

## Describe the feature
Per the [parent issue](https://github.com/dapr/dapr/issues/8706) we should identify an means of updating the Workflow to better enable developers to return data in an activity and support automatically persisting this somewhere and returning only a reference to it and vice versa - given a reference, retrieve the data when an activity is started.

I do think that this should rely on the [proposed binary store building block](https://github.com/dapr/proposals/pull/88) so we have a high-performance mechanism to store arbitrary blobs, using the API shape identified in that proposal.

## Release Note

RELEASE NOTE:

No files, tests, or entry points are named in the issue. Start by reading the linked parent issue and proposed binary store API, then determine the .NET workflow surfaces affected and the required activity input/output reference contract. Done should be defined by an agreed design and corresponding implementation and tests for storing returned data and resolving references.

Written by the indexing model from the issue text.