sqldb: add support for test migrations

Add `migrations_dev.go` and `migrations_prod.go` files which each define
a `migrationAdditions` slice to be appended to the `migrationConfig`
slice. The `migrations_dev.go` file is only built if either the
`test_db_postgres` or `test_db_sqlite` build flags are used.

This slice will be used to add any migrations that are still under
development that we want access to via unit tests and itests but do not
want to expose to our release build.

sqldb/migrations_dev.go

@@ -0,0 +1,5 @@
+//go:build test_db_postgres || test_db_sqlite
+
+package sqldb
+
+var migrationAdditions = []MigrationConfig{}