Add a test case that a Border Router sends a UDP message to Thread device

This test case lets the BR send a UDP message to Thread device using
socket API and verify the message is received on the Thread device via
ot-ctl udp command.

For now this only verifies the direction BR -> ED because the other
direction ED -> BR is depending on platform UDP feature which is not
enabled yet.

Bug: 321168973
Test:  atest ThreadNetworkIntegrationTests:BorderRoutingTest#unicastRouting_borderRouterSendsUdpToThreadDevice_datagramReceived

Change-Id: I590583dd818f3d72dec41184e5427a8b08f842bb