Refactor `ReactiveWebApplicationContext` implementations to align closer
with the `WebApplicationContext` implementations defined in
Spring Framework.

The following classes are now provided:

  - `AnnotationConfigReactiveWebApplicationContext` -- A refreshable
    reactive web context with support for `@Configuration` classes.

  - `GenericReactiveWebApplicationContext` -- A non-refreshable reactive
    GenericApplicationContext.

  - `ReactiveWebServerApplicationContext` --  A non-refreshable reactive
    GenericApplicationContext with support for server discovery.

  - `AnnotationConfigReactiveWebServerApplicationContext` -- A
    non-refreshable reactive `GenericApplicationContext` with support
    for `@Configuration` classes and server discovery.

These classes roughly align to the following Servlet equivalents:

  - `AnnotationConfigWebApplicationContext` (Spring Framework)
  - `GenericWebApplicationContext` (Spring Framework)
  - `ServletWebServerApplicationContext` (Spring Boot)
  - `AnnotationConfigServletWebServerApplicationContext` (Spring Boot)

An additional `ConfigurableReactiveWebEnvironment` interface as also
been introduced, primarily for `@ConditionalOnWebApplication` to use.

Fixes gh-10852