Add HTTP/1.1 request framing enforcer

[gavinbunney]

This adds a Http1FramingEnforcingHandler, immediately after HttpServerCodec in the HTTP/1.1 pipeline, which rejects requests with ambiguous message framing per RFC 9112 section 6.3:

• Transfer-Encoding and Content-Length both present
• Content-Length header appearing more than once
• Content-Length that is not a non-negative integer
• Transfer-Encoding set without chunked as the final coding

This is enabled by default, however can be disabled via zuul.http1.framing.enforcement.enabled as needed.