Support ignoring unknown Praefect migrations

This commit adds an -ignore-unknown flag to the praefect sql-migrate command to make it possible to bypass the unknown migration check. This emulates the behavior of Rails.

We had a canary deployment that ran praefect sql-migrate on a binary that was more recent that the one that was being deployed in production. As a result, the older binary going to production failed in the praefect sql-migrate because it detected a new but unknown migration. This halted the deploy, and we worked around this by rolling back the newly-added tables.

Note that we don't do this by default because the documentation warns that this flag should be used sparingly, and it remains to be seen whether using this mode is a flaw with our deployment process.

Closes #2644 (closed)