This commit improves the existing web reactive configuration
infrastructure with the following changes:
* renamed `WebReactiveConfiguration` to
`WebReactiveConfigurationSupport` and is is no longer a Configuration
class
* created the `WebReactiveConfigurer` interface; Configuration classes
implementing it will augment the web reactive configuration support
* created the `DelegatingWebReactiveConfiguration` and
`WebReactiveConfigurerComposite` to effectively tie those custom-defined
configurers to the main configuration support
* created the `@EnableWebReactive` to active that support in
configuration classes
Issue: SPR-14754
3533024ab8