Add support for nested builders in the DSL for reactive apps

[eleftherias]

Allow spring security for reactive apps to be configured using a nested buider, taking advantage of Java 8 lambdas.

An example configuration would look like this:

http
	.authorizeExchange(exchanges ->
		exchanges
			.anyExchange().authenticated()
	)
	.formLogin(formLogin ->
		formLogin
			.loginPage("/custom-login")
	)
	.redirectToHttps(redirectToHttps ->
		redirectToHttps
			.httpsRedirectWhen(new PathPatternParserServerWebExchangeMatcher("/secure"))
	);

Relates to gh-5557.