[INF-0002] Fill out detailed metadata schema design

[damyanp]

proposals/infra/INF-0002-root-signature-in-clang.md

• The "Metadata Schema" section needs to be filled out with the detailed design for representing root signatures in metadata
• The rest of the document reviewed to ensure it is consistent with this design
• This should also address the open question "what should the named metadata be?"
• Ensure that the validation section includes any validation required on the metadata