[Feature Request] More capabilities on ContinueAsNewWorkflowExecution command

[cretz]

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

Python user at https://github.com/temporalio/sdk-python/issues/103 wants retry_policy for continue as new.

Describe the solution you'd like

Add retry_policy and anything else we think we might want from temporal.api.command.v1.ContinueAsNewWorkflowExecutionCommandAttributes onto coresdk.workflow_commands.ContinueAsNewWorkflowExecution.

Also, please confirm that:

Unless noted otherwise, unset or default field values will re-use the issuing workflow's values.

is accurate for the continue as new command (server doesn't carry these values over for us).