This PR unified the semantics of some workflow APIs.
Those workflow APIs acts on workflow tasks so they could be blocked for a long time. So we have both the blocking and non-blocking versions for them: xxx for blocking and xxx_async for non-blocking APIs.
This PR adds documentation for Workflow Metadata, which we recently added support in https://github.com/ray-project/ray/pull/19372.
Co-authored-by: Yi Cheng <74173148+iycheng@users.noreply.github.com>