Update ITE-10 to reflect actual schema

[nmiyake]

Updates schema values in ITE-10 to reflect the actual values as specified in the in-toto spec and updates the newly proposed field to match the existing style.

[nmiyake]

@adityasaky this is per discussion on https://github.com/in-toto/ITE/pull/50/files#r1651465773.

I personally prefer camelCase as a style and think that the core in-toto layout file format itself could stand to have a revamp of its definition (for example, _type field seems unneeded, and it would be nice to have a full JSON schema definition for the layout file in a single format/place rather than having it spread through the spec), but I think that changing the file format itself would be a separate independent ITE.

In the meantime, given that this ITE is based on the v1.0.0 spec, I think it's confusing for it to refer to the "existing schema" with field name formatting that doesn't match the actual spec.

[adityasaky]

@nmiyake sorry for the lengthy turnaround on this, could you rebase with DCO? Thanks!

[adityasaky]

Thanks @nmiyake!