fix(proxy) keep proxy-authentication request header and proxy-authenticate response header

[bungle]

Summary

It was reported by @ly123-liu on #7722 that Kong removes Proxy-Authentication header from upstream
request.

RFC2612 (https://datatracker.ietf.org/doc/html/rfc2616#section-13.5.1) specifies them as a hop-by-hop
headers that should be removed. But it did not consider use case of having multiple proxies, so RFC7235
corrected the wording https://datatracker.ietf.org/doc/html/rfc7235#section-4.4.

A proxy MAY relay the credentials from the client request to the next proxy if that is
the mechanism by which the proxies cooperatively authenticate a given request.

Issues Resolved

Fix #7722

[fffonion]

Not a blokcer to this PR just a thought:
If I understand the RFC correctly, we are okay to relay it to next proxy/upstream as long as Kong doesn't consume the
proxy-authentication header. But if a proxy does consumes it, it will then the plugin's responsibility to
clear out the header. This indicates we might want a PDK function to properly follow the RFC, something
like header_value = kong.request.consume_hop_by_hop_header("proxy-authentication") and PDK clears
the head before it returns.