[validation-gen] Improve tag parsing utilities

[jpbetz]

#71 reminded that we haven't put all that much rigor into defining how we want tags to be structured. We're got a variety of adhoc parsing strategies that "get the job done" for the cases we've defined so far.

I think it's time we start to consolidate the parsing into a utility. I don't have strong options of how the utility should be structured or what "grammar rules" we should establish, but at the very least, it should be trivial to extract values of different types without having to write parsing code redundantly for each validation plugin.

[jpbetz]

@yongruilin Any interest in taking a pass at this?