Resolves #9695 (WIP code)

Problem

Today, we don't allow people to reuse "snippets'' of YAML across files. We can use YAML anchors but they are limited to a single file. This lead to non-DRY configuration where changing some configuration might require changing multiple files

Solution

Add the ability to type!include <filepath.yml> to include the YAML of filepath.yml in the current section of the code.

e.g. with a (too) simple example

models:

  dbt_project_evaluator_integration_tests:
    +materialized: ephemeral
  dbt_project_evaluator:
    +schema: !include ./incl.yml

and incl.yml being

test_schema_123

This would work with nested YAML as well.

TODO

The attached code worked on my machine with the example above, but this is really a WIP and not ready to ship.
Pending would be :

• adding tests
• fixing mypy issues
• handling partial parsing - right now, if the content of the included file is changed, if partial parsing is on, the new value will not be reflected

Anyone wanting to handle the above is welcome to take over this PR 😄

Checklist

• I have read the contributing guide and understand what's expected of me.
• I have run this code in development, and it appears to resolve the stated issue.
• This PR includes tests, or tests are not required or relevant for this PR.
• This PR has no interface changes (e.g., macros, CLI, logs, JSON artifacts, config files, adapter interface, etc.) or this PR has already received feedback and approval from Product or DX.
• This PR includes type annotations for new and modified functions.