[request] Add pre/post migration SQL hooks

### Is your proposal related to a problem?

I would like to run a set of SQL instructions before and after Hasura runs its migrations.

### Describe the solution you'd like

maybe specific files in the migrations folder that are dedicated to pre/post migrations

for example `hasura/migrations/pre.sql` and `hasura/migrations/post.sql`

Also, presumably `post.sql` should only run when migrations are successful? Or maybe make that an option.

### Describe alternatives you've considered

None so far. Just recently realized I need this.

### If the feature is approved, would you be willing to submit a PR?

No