Fix!: normalize blueprint variables

[georgesittas]

The following model fails to load today, because blueprint variables are not normalized:

MODEL (
  name test_schema.test_model,
  kind FULL,
  blueprints (
    (Test_Var := 1)
  )
);

SELECT
  @test_var AS col

This PR aims to address this issue. I also added a migration script that normalizes all existing blueprint
variable keys in the python_env, unless there are keys that collide when normalized. In that case, we
raise an AssertionError to avoid messing up the state. This allows the user to rename their blueprint
variables in order to avoid this issue and then re-run the migration.