rdp: Allow configuring the refresh rate

We currently hardcode a 60Hz update rate for the rdp backend.

In some cases it may be useful to override this to increase the rate
for a faster monitor, or to decrease it to reduce network traffic.

Co-authored-by: Steve Pronovost <spronovo@microsoft.com>
Co-authored-by: Brenton DeGeer <brdegeer@microsoft.com>
Signed-off-by: Hideyuki Nagase <hideyukn@microsoft.com>
Signed-off-by: Steve Pronovost <spronovo@microsoft.com>
Signed-off-by: Brenton DeGeer <brdegeer@microsoft.com>