Restore Migration.Up and Migration.Down signatures

[bevzzz]

Resolves #1297

In an attempt to support templated SQL migration files #1188 changed the signature for Migration.Up and Migration.Down fields, breaking a part of the migrate package's public contract. This had the effect of breaking all external code that relied on Migrations.Add API previous to this change.

My understanding is that Migrations.Add was never meant to be used outside of the migrate package, Migrations.Register being the officially documented and, I assume, the only intended way for client code to register its Go-based migrations. Based on the reactions to the linked issue, there seems to be a number of users who discovered Add (which is exported) and started using it.

I propose we revert the breaking changes. Both migrate.Migration and migrate.Migrations.Add have been a part of the public API for a long time and breaking them isn't right. The reason internalMigrationFunc was introduced was to support templated migrations and before/after hooks. For each of these the PR presents alternative, b/c-compatible solutions:

1. Migrator can pass template data via context. This makes sense, because it is a piece of data scoped to a single migrate/rollback "request".
2. Before/After hooks are handled in a single place. The code which is calling Up and Down is responsible for applying the hooks, just like it is responsible for calling MarkApplied/MarkUnapplied.

I also added the missing tests for these two features, see testSQLMigrations and testHooks.

[bevzzz]

@vmihailenco if possible, I'd like to include this in the next release, WDYT? Do you agree this is a breaking change worth fixing?

[github-actions]

This pull request has been automatically marked as stale because it has not had activity in the last 30 days. If there is no update within the next 7 days, this pr will be closed. Please feel free to give a status update now, ping for review, when it's ready. Thank you for your contributions!