Add a dedicated IP header to accommodate Java 17 on GAE (#2224)

For reasons unclear at this point, Java 17's servlet implementation on
GAE injects IP addresses (including unroutable private IPs) into the
standard X-Forwarded-For header, which we currently use to embed
registrar IP addresses to check against the allow list. This results in
the server not properly parsing the header and rejecting legitimate
connections.

This PR sets a custom header that should not be interfered with by any
JVM implementation to store the IP address, while maintaining the old
header as a fallback. The proxy will set both headers to allow the
server to gracefully migrate from Java 8 and Java 17 (and potentially
rollback).

Also removed some headers and logic that are not used.

proxy/src/main/java/google/registry/proxy/handler/EppServiceHandler.java

@@ -124,6 +124,7 @@ public class EppServiceHandler extends HttpsRelayServiceHandler {
         .headers()
         .set(ProxyHttpHeaders.CERTIFICATE_HASH, sslClientCertificateHash)
         .set(ProxyHttpHeaders.IP_ADDRESS, clientAddress)
+        .set(ProxyHttpHeaders.FALLBACK_IP_ADDRESS, clientAddress)
         .set(HttpHeaderNames.CONTENT_TYPE, EPP_CONTENT_TYPE)
         .set(HttpHeaderNames.ACCEPT, EPP_CONTENT_TYPE);
     return request;

proxy/src/test/java/google/registry/proxy/TestUtils.java

@@ -100,7 +100,8 @@ public final class TestUtils {
         .set(HttpHeaderNames.CONTENT_TYPE, "application/epp+xml")
         .set("accept", "application/epp+xml")
         .set(ProxyHttpHeaders.CERTIFICATE_HASH, sslClientCertificateHash)
-        .set(ProxyHttpHeaders.IP_ADDRESS, clientAddress);
+        .set(ProxyHttpHeaders.IP_ADDRESS, clientAddress)
+        .set(ProxyHttpHeaders.FALLBACK_IP_ADDRESS, clientAddress);
     if (cookies.length != 0) {
       request.headers().set("cookie", ClientCookieEncoder.STRICT.encode(cookies));
     }