Allow to query for server extensions available. (#591)

Co-authored-by: Jöran Malek <[email protected]>

Author: dkocher

src/main/java/net/schmizz/sshj/sftp/SFTPEngine.java

@@ -108,6 +108,14 @@ public int getOperativeProtocolVersion() {
         return operativeVersion;
     }
 
+    public boolean supportsServerExtension(final String extension, final String domain) {
+        return serverExtensions.containsKey(extension + "@" + domain);
+    }
+
+    public String getServerExtensionData(final String extension, final String domain) {
+        return serverExtensions.get(extension + "@" + domain);
+    }
+
     public Request newExtendedRequest(String reqName) {
         return newRequest(PacketType.EXTENDED).putString(reqName);
     }