[UNREAL] Set up Server Boot map in Beamball sample

[PedroRauizBeamable]

To exemplify the case where you enter PIE in your Server Boot (a map designed to wait until players are connected and them "Server Travel" them to the actual gameplay map, we should set this up.

This would be your dedicated server build's default map and it'll have a regular InitSDK call in it. To add PIE support to it, we should add a "Beam Prepare PIE" node that will wait for our PIE Fake Lobby to be set up before allowing people to come in.