Provide a way to configure payload size for UDP probes

[manugarg]

Francisco Castello on Slack:
"I am using UDP prober with server and client running on different VMs and I was wondering if there is a way to make the UDP packets bigger? I did a pcap and the packets are 148 bytes. Is there a way tp add padding to the payload.
We have some cases we want to make the packet bugger than the MTU as we have had fragmentation issues in the past with UDP so I wondered if there is a way of doing that.

CC: @ls692

[manugarg]

Assigning this to myself for now. I'll discuss with Lenin (@ls692) before making any substantial progress.

[manugarg]

Fixed by 6bb4918.