Add general purpose Vehicle plugin + minor fixes

[RachithP]

• Adds general purpose Vehicle plugin
• Use OpenBot V1 versioned assets
• Removes OpenBot plugin
• Updates getting started example to work with SphereAgent and VehicleAgent
• Minor fixes

[mikeroberts3000]

• Let's just call it OpenBot instead of OpenBot_V1.
• Remove Master_Materials folder and just go with a flat structure inside Materials.
• Make sure to "fix up redirectors" in the editor after renaming anything (https://docs.unrealengine.com/5.2/en-US/asset-redirectors-in-unreal-engine/).