[CT-2121] Make model contracts agnostic to ordering

[MichelleArk]

The current model contract comparison (assert_columns_equivalent macro) validates that the user-provided column names and data types are identical: in name, type, and order the column names obtained from the model SQL.

However, it will be cumbersome for users to ensure that the order of configured columns in a model yaml file always matches with the order the database provides.

For example, the SQL model configured with the schema below fails its contract check:

-- model.sql
SELECT 1 as a, 2 as b

-- schema.yml
models:
- model: 
  contract: True
  columns:
    - name: b
       data_type: INT
    - name: a
       data_type: INT

The equality comparison is done directly on formatted columns from the SQL + schema yaml, and could instead be done on the intermediate collection of unordered columns.

Relevant discussion: #6271 (comment)

Note: should not be merged before #7064, as this ordering check is currently done in assert_columns_equivalent and prevents constraint ddl generation from being generated in an invalid order.

[gshank]

Note: #7064 cannot be merged before this because the column checks would fail. These two tickets will processed in one pull request.