Add flag to disable "Non-zero priority for non-ternary match" check in BMv2

[ghost]

It's hard for us right now to only set priorities for ternary matches (we should be able to be compliant eventually though). Could you add a flag to Bmv2 that disables the check which validates that priorities are only set for ternary matches? The line you would have to modify is:

//depot/google3/third_party/p4lang_PI/proto/frontend/src/device_mgr.cpp:1740
RETURN_ERROR_STATUS(Code::INVALID_ARGUMENT, "Non-zero priority for non-ternary match");

I would send you a pull request, but I don't quite know how flags work in BMv2.