ENH: add structured data support

[tacaswell]

This builds on bluesky/tiled#66 and bluesky/event-model#215

[tacaswell]

bundle = event_model.compose_run()
start_doc, compose_descriptor, compose_resource, compose_stop = bundle
assert bundle.start_doc is start_doc
assert bundle.compose_descriptor is compose_descriptor
assert bundle.compose_resource is compose_resource
assert bundle.compose_stop is compose_stop
motor_data = 5.0
counter_data = 10
image_data = numpy.zeros((512, 512))
structure_data = numpy.zeros((512,), dtype=np.dtype("f,i"))
bundle = compose_descriptor(
    data_keys={
        "motor": {"source": "...", **infer_datakeys(motor_data)},
        "counter": {"source": "...", **infer_datakeys(counter_data)},
        "structure_data": {"source": "...", **infer_datakeys(structure_data)},
        "image": {
            **infer_datakeys(image_data),
            "source": "...",
            "external": "FILESTORE:",
        },
    },
    name="primary",
)
descriptor_doc, compose_event, compose_event_page = bundle

structure_from_descriptor(descriptor_doc, "data", 5, {})

is the snippet I tested this with.