This commit separates the execution of SQL and in-code migrations
from their construction. This change is necessary because,
currently, the SQL schema is migrated during the construction
phase in the lncfg package. However, migrations are typically
executed when individual stores are constructed within the
configuration builder.
This commit introduces support for custom, in-code migrations, allowing
a specific Go function to be executed at a designated database version
during sqlc migrations. If the current database version surpasses the
specified version, the migration will be skipped.
