igvm: meta tracking issue for CoRIM support

[chris-oo]

There's more work to do for CoRIM support than just defining the header in igvm. This issue tracks the overall workstream and remaining work to do, to be updated as we discover more things required.

1. igvm_defs and basic support for corim documents and signatures: igvm_defs, igvm: introduce corim measurement header #99
2. CBOR validation of payload and spec compliance with each given platform. This should also update the docs with links to public documentation for each platform's CoRIM type.
3. Figure out how to implement measurement generation and corim header generation as part of the core igvm crate. This may be tricky as there may be additional things that consumers need to add to the corim, on top of the platform required ones. At a minimum, the serialize/file builder api may need to move towards a mode where you can dynamically add headers at runtime, that will then get checked to make sure they are valid with the other headers in the file.