HttpSecurity construction

It'd be handy to be able to construct an instance of HttpSecurity independently from an WebSecurityConfigurerAdapter.

In theory, this is possible since HttpSecurity has a public constructor, but that's currently impractical due to its abstract nature:

public HttpSecurity(ObjectPostProcessor<Object> objectPostProcessor,
			AuthenticationManagerBuilder authenticationBuilder,
			Map<Class<?>, Object> sharedObjects)

If there were a simpler way to build an HttpSecurity instance, then code like the following would be within reach:

Map<String, Filter> proxies = new HashMap<>();
// ...

String tenant = resolveTenant(request);
Filter proxy = proxies.computeIfAbsent(tenant, k -> {
    HttpSecurity http = // construct
    // configure by tenant
    return new FilterChainProxy(http.build());
});
proxy.doFilter(request, response, chain);

which seems like a powerful tool for multi-tenancy.

Start by reviewing HttpSecurity's public constructor and its relationship to WebSecurityConfigurerAdapter. Define what supported independent construction should look like for the multi-tenant FilterChainProxy example, including how configuration and HttpSecurity.build() would work; done means the requested construction path is implemented and usable as shown.