This commit introduces a "ReactiveHealthIndicator" contract that can be
implemented for health checks against a reactive API.

When running in a WebFlux-based web app, the health and status endpoints
transparently use this in a WebFlux-based application and regular
HealthIndicator are executed on the elastic scheduler.

When running in a Servlet-based web app, the endpoints includes and
adapts available ReactiveHealthIndicators automatically

Closes gh-7972