Spring Add minimal Kotlin DSL RouterFunction attributes support

Add minimal Kotlin DSL RouterFunction attributes support

Fixes https://github.com/spring-projects/spring-framework/issues/28567.

The PR is composed by 2 commits that are more easy to review one by one : 1. Add test case on nested RouterFunction attributes : as nested route attributes are already supported but not clearly tested 2. Add minimal Kotlin DSL RouterFunction attributes support : just minimal to ensure that minimal support will be quickly integrated ; I think better Kotlin integration could be done be should probably be discussed

Comment From: poutsma

This PR has been merged, see https://github.com/spring-projects/spring-framework/commit/283bc9ede936bd41b68ad2ee7fe00047d5550b93.

@christophejan Thank you for submitting a PR.