Muons: set run "comment" from bluesky scan

As a user on muons I want to be able to find what scan a DAE `.nxs` file was part of.

To do this, insert the bluesky scan ID (or another suitable string identifier) as the "run comment" in the experiment details. This way we have a two-way link between `.nxs` which has a pointer to the scan it's part of, and the scan which has pointers to the `.nxs` it generated.

We can write this as a bluesky preprocessor that reacts to `open_run` and `close_run` messages. After we've seen the `open_run` message we know the UID of a scan, the preprocessor can then write this to the comment field. On close run it should put the comment back to whatever it was before.