Add a new `WebServerApplicationContext` interface that provides a common
abstraction for all application contexts that create and manage the
lifecycle of an embedded `WebServer`.

Allows server namespaces to become a first-class concept (rather
subverting `ConfigurableWebApplicationContext.getNamespace()`) and
allow us to drop `getServerId()` from `WebServerInitializedEvent`.

Also helps to improve `ManagementContextAutoConfiguration` and
`ManagementContextFactory`.

Fixes gh-11881