Codify "at least one English entry" requirements for CNA descriptions

[chandanbn]

ensure no more than one entry for any language code.

[chandanbn]

enforce this for the cna container in published sub-schema, and one in rejected sub-schema.

[chandanbn]

To ensure no more than one entry for any language code, we may have to use a non-standard keyword that works when the schema is validated with ajv (on cve-services):
https://github.com/ajv-validator/ajv-keywords#uniqueitemproperties

[rsc]

The PR I just posted, for requiring English, is standard JSON schema.
I agree that it appears that checking for unique item properties requires an extension.