Add high-level workflow describe

[cretz]

Describe the solution you'd like

• Add high-level workflow describe call
  • Probably describe on the workflow stub
• Return a WorkflowExecutionDescription that contains at least the things list does and access to the raw describe response for now
  • In other SDKs, this would be an extension of the WorkflowExecutionMetadata class since it has all of that and more
• Make sure it is interceptable

See TypeScript, Python, and .NET SDKs if looking for design inspiration for this call, or reference temporalio/sdk-go#1667.