Add a way to disable ProxyExchange hostname verification

I'm attempting to use gateway to bypass a load balancer and proxy a prometheus scrape request.
I'm hitting an IP address directly, and setting the host header that the server is expecting.

This causes ssl issues.

I attempted to set spring.cloud.gateway.httpclient.ssl.use-insecure-trust-manager=true which got me past the initial error, but it now fails hostname verification:

  Caused by: java.security.cert.CertificateException: No subject alternative names matching IP address 1.2.3.4 found
     at sun.security.util.HostnameChecker.matchIP(HostnameChecker.java:168) ~[na:1.8.0_162]
     at sun.security.util.HostnameChecker.match(HostnameChecker.java:94) ~[na:1.8.0_162]
     at sun.security.ssl.X509TrustManagerImpl.checkIdentity(X509TrustManagerImpl.java:455) ~[na:1.8.0_162]
     at sun.security.ssl.X509TrustManagerImpl.checkIdentity(X509TrustManagerImpl.java:436) ~[na:1.8.0_162]
     at sun.security.ssl.X509TrustManagerImpl.checkTrusted(X509TrustManagerImpl.java:252) ~[na:1.8.0_162]
     at sun.security.ssl.X509TrustManagerImpl.checkServerTrusted(X509TrustManagerImpl.java:136) ~[na:1.8.0_162]
     at org.cloudfoundry.security.FileWatchingX509ExtendedTrustManager.checkServerTrusted(FileWatchingX509ExtendedTrustManager.java:73) ~[container_security_provider-1.11.0_RELEASE.jar:1.11.0.RELEASE]

Start with ProxyExchange and the existing spring.cloud.gateway.httpclient.ssl.use-insecure-trust-manager property described in the issue. Trace how the gateway performs TLS hostname verification, then verify that hostname verification can be configured independently and that the documented behavior covers requests sent to an IP address with a custom Host header.