build_info: we should have a way to add arbitrary data

[justus-camp-microsoft]

From discussion about #1537, ideally we wouldn't have to make an oss change every time we want to add some arbitrary data from an internal build

[chris-oo]

I have some ideas on if we could define some sort of manifested format, that we would expose as a child node of this node, that we would consume via build.rs.

[chris-oo]

I think even having stuff such as "this is a release or debug build" is useful in oss, too. There's not an easy way to tell.