Add simple player simulation, fix terrain block search

[JerritEic]

Summary of changes:

• Added simulation-based player emulation
  • Supports two basic behaviors, BoundedRandom and FixedDirection
• Added player simulation, terrain, and networking parameters as command line arguments
  • See README.md for updated list of supported arguments
• Refactored terrain layer management
  • Now groups of layers can be accessed through a TerrainBank
  • Two layer groups added, Flat and Default
• Converted terrain modification and access to IJobEntity to support parallel scheduling and burst compilation
• Fixed spawn location randomness, added a system to search deterministically for a valid spawn location
• Refactored split between Opencraft 2 and Polka
  • Polka now exclusively manages Player prefabs, Opencraft 2 accesses them solely through Polka's connectionPlayerObjects dictionary.
  • Polka now exclusively handles command line arguments, but supports extensions to its argument list through the ArgumentClass attribute…d movement