[Feature] new configuration to run tests on only the "new" data for snapshots and incremental models

[graciegoheen]

Describe the feature

A more general solution for #10236 and #10864

Folks want to be able to test just their "new" data before it's inserted into their existing table. This is relevant for:

• snapshots
• incremental models

For example, you may want to test that your unique_key is actually unique for the "new" data. If that test fails don't insert the "new" data into the existing table.

We should create a new data test configuration to allow you to configure a given data to only run on the "new" data.

models:
  - name: my_model
    columns:
      - name: id
        data_tests:
          - unique:
              new_records_only: true  # name/spec TBD

Acceptance criteria:

• you can set this config on a data test for an incremental model
• you can set this config on a data test for a snapshot
• when set, the data test will only run on "new" data before insert/merge
• if data test fails, the subsequent insert/merge will be skipped