Add commit hash to ActsConfig.cmake

This makes it much easier to figure out which exact commit this build came from.

The new output in a client package after find_package(Acts) is then like

-- found Acts version 0.6.0 - [30571fcb] - DIRTY
-- supported components are:
--    - Core
--    - Legacy
--    - IdentificationPlugin
--    - DD4hepPlugin
--    - DigitizationPlugin
--    - MaterialMappingPlugin
--    - TGeoPlugin
--    - JsonPlugin
-- loading components:
--    - Core
--    - Legacy
--    - MaterialMappingPlugin
--    - IdentificationPlugin
--    - TGeoPlugin
--    - DD4hepPlugin
--    - DigitizationPlugin
-- Disable component 'external/acts-fatras'
-- Disable component 'Detectors/DD4hepDetector'
-- Disable component 'Plugins/Geant4'
-- Disable component 'Plugins/Pythia8'

@msmk: I'm using a cmake script I found, it's licensed under the boost license. Do you think I should mention this in the readme?

Closes ACTS-520