Missing metadata from kernelspec model

[declanvk]

Hello,

Currently the documentation and the /api/kernelspecs endpoint have a field for metadata included in a kernelspec. However, that field is not included in the ISpecModel.

I also noticed that while the env field exists on the ISpecModel, the validation code in validate.ts does not check if the env is present on the incoming message and never adds it to the model.

I propose to add metadata to ISpecModel + validation code, and env to the validation code.

@dleen @blink1073

[declanvk]

@meeseeksdev "pkg:services"