.NET Declarative Workflows - Update root element usage

[crickman]

Currently using AdaptiveDialog for root workflow element. Prior decision point was to introduce a Workflow element used as the root for Foundry Workflows.

This involves:

1. Updating the Copilot Studio Object Model to define the Workflow elment.
2. Updating agent-framework dependency on the Copilot Studio Object Model
3. Updating code in DeclarativeWorkflowBuilder

Sample workflows required update at (https://github.com/microsoft/agent-framework/tree/main/workflows).