SBOM JSON result: add `$schema`

[jkowalleck]

requires https://github.com/CycloneDX/specification/issues/106

---

the current JSON result follows an existing schema, described in https://github.com/CycloneDX/specification/tree/master/schema

feature request: add the used schema as a property $schema of the resulting json. the value must be the $id of the schema used.

spec	schema-id
1.2	http://cyclonedx.org/schema/bom-1.2a.schema.json
1.3	http://cyclonedx.org/schema/bom-1.3.schema.json

example:

{
  "$schema": "http://cyclonedx.org/schema/bom-1.3.schema.json",
  "bomFormat": "CycloneDX",
  "specVersion": "1.3",
  "serialNumber": "urn:uuid:3e671687-395b-41f5-a30f-a58921a69b79",
  "version": 1,
  "components": [
    {
      "type": "library",
      "name": "acme-library",
      "version": "1.0.0"
    }
  ]
}