DEV: Prepare new structure for migrations-tooling (#26631)

* Moves existing files around. All essential scripts are in `migrations/bin`, and non-essential scripts like benchmarks are in `migrations/scripts`
* Dependabot configuration for migrations-tooling (disabled for now)
* Updates test configuration for migrations-tooling
* Shorter configuration for intermediate DB for now. We will add the rest table by table.
* Adds a couple of benchmark scripts
* RSpec setup especially for migrations-tooling and the first tests
* Adds sorting/formatting to the `generate_schema` script

migrations/README.md

@@ -0,0 +1,11 @@
+# Migrations Tooling
+
+## Development
+
+### Running tests
+
+You need to execute `rspec` in the root of the project.
+
+```bash
+bin/rspec --default-path migrations/spec
+```