Add MultiTenantAuthenticationManagerResolver

[jzheaux]

There ought to be a simple way to configure multi-tenancy for common use cases like by domain, by path, and by header.

Being able to do something like

@Bean 
public AuthenticationManagerResolver<HttpServletRequest> byHeader
        (Converter<String, AuthenticationManager> lookupByTenant) {
    return DefaultAuthenticationManagerResolver.resolveByHeaderValue
            ("X-Tenant-Id", lookupByTenant);
}

might be nice.

[rwinch]

Some thoughts...perhaps create DefaultAuthenticationManagerResolver which allows injecting a Converter for the request to be converted into an id and then another Converter for the id to be converted into a manager. Then static factory methods on it that resemble byHeader as shown above

[jzheaux]

We could achieve the same thing with:

static <T> AuthenticationManagerResolver<HttpServletRequest> resolveByRequestConverter
        (Converter<HttpServletRequest, T> requestConverter, 
                Converter<T, AuthenticationManager> resolver)

However, I do like the idea of a concrete implementation that can hold state. I've updated the PR accordingly.