Generate Decision_Point.schema.json using Pydantic

[ahouseholder]

• child of Consider use of pydantic built-ins to generate JSON schemas from python data classes #702

Describe the solution you'd like

Back-port the fields that are found in the hand-built JSON Schema

https://github.com/CERTCC/SSVC/blob/main/data/schema/v1/Decision_Point-1-0-1.schema.json

to create a functional match using Pydantic's JSON schema feature set.

The net result should be that the output of

from ssvc.decision_points.base import SsvcDecisionPoint
import json
print(json.dumps(SsvcDecisionPoint.model_json_schema(),indent=2))

is as close as possible to https://github.com/CERTCC/SSVC/blob/main/data/schema/v1/Decision_Point-1-0-1.schema.json