Stage eiger asynchronously in rotation plan

[DominicOram]

Fixes DiamondLightSource/mx-bluesky#206

Makes use of the start_preparing_data_collection_then_do_plan plan stub we have for setting up the collection that we use in the XRC. begin_sample_environment_setup will ultimately be fully removed by DiamondLightSource/mx-bluesky#205

To test:

1. Confirm tests pass
2. Manually confirm grid_detect_then_xray_centre_composite still passes

[olliesilvester]

I will make a PR for this branch which fixes the merge conflict, as well as makes similar changes for the multi_rotation_scan, eg make it also stage the eiger as early as possible

Going to hold off until hyperion mx_bluesky merge is sorted actually

[DominicOram]

Replaced by DiamondLightSource/mx-bluesky#190