Add command: DescribeExternalWorkflowExecution

[lorensr]

Is your feature request related to a problem? Please describe.

In order to describe an external workflow, you currently need to call workflow.describe() using a client in an activity.

Describe the solution you'd like

Being able to describe an external workflow from inside a workflow.

Additional context

User request: https://github.com/temporalio/sdk-typescript/issues/845