Allow migrations run to be recorded
Allow migrations run to be recorded

For now, nothing is recorded. But this allows to create
implementations of Main and Copybara that can record the migrations
that were run.

PiperOrigin-RevId: 168004635
Change-Id: I324f6f44642aa2a61f87e8c1e57ed3c603e8b19b

google/copybara,google/copybara,google/copybara