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 "subaruglobal" to "subaru_"

Reference PR which added Subaru Global checksum support: https://github.com/commaai/opendbc/commit/a57e7ddbd72c92241d5d6442da9d47c55e95a8cf

[martinl]

Added initial setup with mock checksum function https://github.com/martinl/opendbc/commit/362cc6c7c535fe237227187f4b1d017fb87226ba