Aggregated API Server Development overview

Building custom Kubernetes aggregated API servers that register with kube-apiserver via APIService resources and serve custom API groups backed by non-etcd storage. Covers the apiserver library (k8s.io/apiserver), REST storage interfaces, codec and conversion chains, request handling, admission webhooks in the aggregated context, table conversion for kubectl output, watch semantics with external stores, and health/readiness probes. Includes performance considerations for backing the aggregated server with PostgreSQL, MySQL, or other relational databases.