Add vehicle configuration

[Gamenot]

• [x] Enable composed configuration of vehicles
  • [x] urdf dynamics model
  • [x] tire params
  • [x] controller params
  • [x] chassis params
  • [x] visual model
  • [x] vehicle type
• [x] Add heavy truck physics model
• [x] Add heavy truck visual model
• [x] Add light truck physics model
• [x] Add SMARTS_ASSETS_PATH
• [x] Move smarts.core.models to smarts.assets.
• [x] Enable configuration substitution in smarts configuration files.
• [x] Use urdf chassis size as Ackermann vehicle size.
• [x] Fix tests.

Closes #2120 #2116

[Gamenot]

test-docs failing is expected until the smarts.assets module exists.