Move `ApplicationHome`, `ApplicationPid` and `ApplicationTemp` to the
`system` package. Since `system` package is now much lower level, the
existing `FileWriter` implementations also needed to move to prevent
package tangles.
Fixes gh-8614
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
Andy Wilkinson
Phillip Webb
Madhura Bhave