[crashpad] support an override config

DX-707 #comment #done
DX-714 #comment #done

TESTED=`fx set --packages garnet/packages/config/crashpad_upload_to_prod_server ...` then `crasher` (report id a2c9dc128f7f1fb1)

Change-Id: I3ba118c8af03356ca2a51751eb4f15f46fc1b06b
5 files changed