Investigate using custom JSON parser

[haydentherapper]

Description

Context threads:

• Deserializing into spec.Swagger is almost 20x slower than deserializing into map[string]interface{} kubernetes/kube-openapi#315
• update kube-openapi kubernetes/kubernetes#112988
• openapiv2 experimental json unmarshaler kubernetes/kube-openapi#319

OpenAPI's JSON unmarshalling is quite slow, and there's been some efforts in the k8s community to create a more performant marshaller. We should see if we can integrate this into Rekor to improve performance.