Add API extension for Knative v0.19.0

[manusa]

Generate Java API client and model types for Knative (Kubernetes-based platform to deploy and manage modern serverless workloads).

See also:

• https://github.com/knative/serving/releases/latest
• https://knative.dev/docs/install/any-kubernetes-cluster/

[manusa]

Support was added in:

• https://github.com/manusa/yakc/commit/7796d5031e19e55d445b7942eba7088df99ccd7b