Generalise path flagging to metadata?

[MattWindsor91]

We now have information about paths that we want to carry through the payload that aren't just the path and its flags:

• intended kind;
• block restrictions;
• eventually maybe other things.

The more metadata that is stored with the path, the more checking the path consumer can do to make sure the path producer is working (and, later on, any reducer is working).

This issue suggests generalising Path_flag.Flagged.t to contain multiple different kinds of path metadata. We can begin by renaming, then add things, then add consumer checking support, in stages.