Add fsetstat variant of SetExtendedData

Signed-off-by: Peter Verraedt <peter@verraedt.be>

client.go

@@ -2058,6 +2058,28 @@ func (f *File) Chmod(mode os.FileMode) error {
 	return f.c.fsetstat(f.handle, sshFileXferAttrPermissions, toChmodPerm(mode))
 }
 
+// SetExtendedData sets extended attributes of the current file. It uses the
+// SSH_FILEXFER_ATTR_EXTENDED flag in the setstat request.
+//
+// This flag provides a general extension mechanism for vendor-specific extensions.
+// Names of the attributes should be a string of the format "name@domain", where "domain"
+// is a valid, registered domain name and "name" identifies the method. Server
+// implementations SHOULD ignore extended data fields that they do not understand.
+func (f *File) SetExtendedData(path string, extended []StatExtended) error {
+	f.mu.RLock()
+	defer f.mu.RUnlock()
+
+	if f.handle == "" {
+		return os.ErrClosed
+	}
+
+	attrs := &FileStat{
+		Extended: extended,
+	}
+
+	return f.c.fsetstat(f.handle, sshFileXferAttrExtended, attrs)
+}
+
 // Truncate sets the size of the current file. Although it may be safely assumed
 // that if the size is less than its current size it will be truncated to fit,
 // the SFTP protocol does not specify what behavior the server should do when setting