fix(migration): Snapshot all struct types (#3211)

declaring all migration-irrelevant types as []byte may erase data
for repeated protobuf fields. we hoped that "obscuring" the original
types of these columns with []byte would pass-through their values
unchanged, allowing us to omit entire types (and their descendent
trees) that were not being operated on in a migration. Fewer imported
types would lead to a more expressive migration file whose intent was
more clear to the reader. We will need to experiment with other ways
to achieve this in the future.

fixes #3200