chore(grouping): Add project_root to debug_meta schema

[lobsterkatie]

Draft until we decide if this is the right approach. (In other words, ignore the questions I asked below for now.)

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As part of a larger bug fix, we are in the midst of trying to move in_app logic from the Python SDK to the sentry server. To do that, we need to send the app's root path in the event, which we are for now planning to include in debug_meta.

In my testing, I discovered that even though the debug_meta schema includes an other entry with the note /// Additional arbitrary fields for forwards compatibility, in order for the project root data to make it all the way to the grouping algorithm it has to survive the normalizer, and to do that, it has to be an explicitly listed. (Reviewers, please correct me if I'm wrong about this.) This PR does that, and adds a note explaining the requirement for any future additions.

Note to reviewers: I got the metastructure values from the abs_path entry, to which the project_root will get compared. Happy to change any of them if needed. I also didn't add or modify any tests related to this change. If that's needed, I would appreciate some guidance as I've never worked in rust before.