igvm: meta tracking issue for CoRIM support

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: #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.

Start by reviewing the igvm_defs and basic CoRIM support in #99, then map the remaining work across CBOR validation, platform-specific compliance and documentation. The core igvm crate and its serialize/file builder API are the stated entry points for measurement and header generation. Done means the remaining validation, documentation and generation work is defined and implemented, including compatible runtime header handling.