Health endpoint reporting the initialization state, and an option to initialize eagerly at startup

Kubernetes readiness and liveness probes need an endpoint that answers without waiting for XWiki to initialize and tells whether the instance is starting, running data migrations, running the Distribution Wizard, ready or failed. Today /rest blocks until initialization completes and view URLs return 202 while initializing.

Proposal: a /health servlet (JSON, 200 when ready, 503 otherwise) aggregating readiness indicators provided by oldcore and extension-distribution, plus a core.initialization.eager property to start the initialization at servlet context start instead of at the first request.