Add Subaru pre-global checksum support to OpenDBC

[martinl]

• implement Subaru pre-global checksum in opendbc
  • python implementation in subaru-PR-test subarucan: https://github.com/martinl/openpilot/blob/subaru-PR-test/selfdrive/car/subaru/subarucan.py#L7
• implement checksum type detection
• change DBC filter back from "subaru_global_" to "subaru_"

Reference PR which added Subaru Global checksum support: commaai/opendbc@a57e7dd

[martinl]

Added initial setup with mock checksum function martinl/opendbc@362cc6c