Add referenced JSON subschemas to repository

[david-waltermire]

The 5.0 JSON format references a few external JSON schema. These should be added to the repository to allow the referenced schemas to be resolved.

For example:

"cvssV3_1": {
    "$ref": "file:cvss-v3.1.json"
},
"cvssV3_0": {
    "$ref": "file:cvss-v3.0.json"
},
"cvssV2_0": {
    "$ref": "file:cvss-v2.0.json"
}

Should be updated to resolvable locations.

[david-waltermire]

The schemas can be found here:

• https://www.first.org/cvss/cvss-v2.0.json - JSON Schema for CVSS v2.0
• https://www.first.org/cvss/cvss-v3.0.json - JSON Schema for CVSS v3.0
• https://www.first.org/cvss/cvss-v3.1.json - JSON Schema for CVSS v3.1

Should we reference these on the FIRST site using the URLs above, or import a copy into this repo?