Feat!: add model blueprinting

[georgesittas]

This PR adds a new feature: the ability to implement model blueprints based on a single template.

This feature is useful when multiple models need to share the same logic, but parameterized differently. Today, this is generally possible by either duplicating model definitions, thus violating the DRY principle, or creating a custom loader, which involves knowing the inner workings of the codebase.

A simple example that demonstrates the syntax to achieve model blueprinting is shown below:

MODEL (
  name @blueprint.some_table,
  blueprints (
    (blueprint := customer1),
    (blueprint := customer2),
    (blueprint := customer3),
  ),
);

SELECT
  *
FROM @blueprint.some_source

This definition essentially expands into three different models, each one sharing the same logic but using a different value for the @blueprint variable. A blueprint can be further customized by specifying additional variables:

MODEL (
  name @customer.some_table,
  blueprints (
    (customer := customer1, field_a := x, field_b := y),
    (customer := customer2, field_a := z, field_b := w),
  )
);

SELECT
  @field_a,
  @field_b
FROM @customer.some_source

Similarly, this will expand into two different models, where each one will contain the variables specified in the corresponding blueprints tuple. For example, the first model would look like:

MODEL (
  name customer1.some_table,
);

SELECT
  x,
  y
FROM customer1.some_source

The blueprint variable mappings can be combined with custom macros to implement even more complicated patterns, e.g., where the model logic itself changes completely depending on a variable's value.