i15-1: Create a load and centre plan

Following https://github.com/DiamondLightSource/crystallography-bluesky/pull/76 we have the start of a sample alignment plan but this probably makes sense to combine with the robot load so that it becomes a plan that puts a sample from the table into data collection position. Additionally, from the description given by the scientists:

1. fast shutter closed
1. move Eiger to [10 degrees]
1. robot load sample
1. spin on
1. set attenuation to a small value? [1% to be safe?]
1. open fast shutter
1. measure [100] points at [0.1s] each from hz [+20 to -10 mm]
1. close fast shutter
1. Some logic of where the best position is (maybe if sample is present at 0 position, go there, if not go to the centre of most continuous stretch of sample? (Think spinning, moving powders)
1. move to optimal position

we are missing points 2, 4 and 5.

## Acceptance Criteria
- We can do a full `load_and_centre` as specified above
- The individual robot load/sample centre plans are kept for testing

Start by reading pull request 76 and the existing robot-load and sample-centre plans. Trace how those individual plans are invoked and identify the missing shutter, Eiger, spin, attenuation, scan, and positioning steps. Done means a full load_and_centre plan follows the listed sequence while preserving the individual plans for testing.

Written by the indexing model from the issue text.