Factor out bluesky hints-parsing from Best-Effort Callback

[danielballan]

The "hints" feature of bluesky was prototyped in the Best-Effort Callback. We are revisiting and expanding it, with the eye toward connecting to Xi-cam's workflow management. This work was being done in the (now-archived) bluesky-browser; it has been ported into the scoped-down bluesky-mpl package.