[CT-2751] Support Configuration and Enabling/Disabling Semantic Models

[peterallenwebb]

Acceptance Criteria

• A semantic_model can be enabled and disabled by YAML configuration, both individually and at the project level.
• A semantic_model can be configured to belong to a group, both individually and at the project level, so that it can ref a private model within the same group.

# models/semantic.yml
```yml
semantic_models:
  - name: orders
    config:
      enabled: true | false
      group: some_group

# dbt_project.yml
semantic_models:
  my_project_name:
    +enabled: true | false
    +group: some_group

[jtcohen6]

@peterallenwebb I missed this one a few weeks ago! What are the implications of inclusion in the Project class? Is this something we need to get in before cutting the v1.6 RC on Thursday?

Update: Needed to support configuring semantic models, including enabled: true|false. Peter will update title + issue description with definition of done

[jtcohen6]

@emmyoop I updated the description, to reflect some findings from my own local testing:

• currently semantic_models do not accept a config property at all in their yaml definitions
• we'd also want to support group as a config, in addition to enabled, so that semantic models could reference a private model defined within the same group