Summary:
Adding if and while control ops to brew, also adding unit tests
Note: unlike net_builder where we can figure which blobs are external and which ones are local to subnets, here in brew we need to use external_blobs param explicitly to point at external blobls
Reviewed By: harouwu
Differential Revision: D6440508
fbshipit-source-id: c920f0af84b77ccb2d8462ffc7567bb1908c844a
