xds: use UNKNOWN for auth algorithm type during per-rpc authority verification (grpc#12421)

While we can get the cipher suite name with
`sslEngine.getHandshakeSession().getCipherSuite()`, for the `authType`
to use in `X509ExtendedTrustManager.checkServerTrusted` it needs to go
through a mapping, for example, for the cipher suite name
"TLS_ECDHE_RSA_WITH_AES_128_GCM_SHA256" the `authType` to use is
actually `ECDHE_RSA`. (JDK code maintains such a
[mapping](https://github.com/openjdk/jdk/blob/844118a9d854459778f88d299b148c2288131344/src/java.base/share/classes/sun/security/ssl/CipherSuite.java#L113)).
Since we don't have all this information handy to use, and UNKNOWN for
`authType` works and has actually been observed being used during Tls
handshake, we are using the same during the per-rpc authority
verification check as the Tls connection has already been established by
then.

Author: kannanjgithub

netty/src/main/java/io/grpc/netty/X509AuthorityVerifier.java

@@ -103,6 +103,6 @@ private void verifyAuthorityAllowedForPeerCert(String authority)
       throw new IllegalStateException("checkServerTrustedMethod not found");
     }
     checkServerTrustedMethod.invoke(
-            x509ExtendedTrustManager, x509PeerCertificates, "RSA", sslEngineWrapper);
+            x509ExtendedTrustManager, x509PeerCertificates, "UNKNOWN", sslEngineWrapper);
   }
 }