Consider supporting expression arguments for vet

[blotero]

By looking at the source code, I can see the pycue APIS are currently supporting the following arguments:

• Schema source (cue file)
• Target validation

However, there is a very useful argument from cue vet which is -d or --schema which allows evaluation of non cue files against a particular expression, for example:

cue vet schema.cue users.json -d "[...#User]"

Which is very useful in several use cases.

[tebeka]

Thanks. I'll have a look when I find some time. Note that this project is not well maintained. I suggest you'll look at https://github.com/cue-lang/cue-py which is maintained by the cue team.