Move "x_generator" to an official and optional string value "generator"

[jayjacobs]

I suggest, as a way to standardize this field across different tools and more importantly, json records, that we add the "generator" field as a string into the "cveMetadataPublished" definition, and move away from the experimental field for the same purpose.

This is related to, but independent from #318

To be inserted into the properties of the cveMetadataPublished definition (and not required):

generator: {
    type: "string",
    description: "An identifying string for any software used to generate or maintain this record, e.g. \"Vulnogram 0.2.0\".",
    minLength: 1,
    maxLength: 256
},

And leave the field as optional.