I added shared base classes to all of the Fee extension types that
make it possible to fully ignore the version in the flows. (You
ask for a FeeCreateCommandExtension, for example, and you get one
without having to worry about which). This is an improvement over
the old code that asked you to provide a list of possible fee
extensions and then ask for the first one in preference order.
As part of this I was able to make the Fee implementation a bit
simpler as well.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=137992390
