Add high-level workflow describe

[cretz]

Describe the solution you'd like

• Add a DescribeWorkflow call on client
  • Probably deprecate DescribeWorkflowExecution
• Return a WorkflowExecutionDescription that contains some obvious things and access to the raw describe response for now
  • Ideally this should embed the type that just represents the workflow information that can come back from list even though we don't have high-level list call at this time
• Make sure it is interceptable

See TypeScript, Python, and .NET SDKs if looking for design inspiration for this call, or reference temporalio/sdk-java#2265.