[Feature] define groups in root-level yml file

### Describe the feature

Currently, you can define groups in any “content” yml file (models/, seeds/, etc), but not as a root-level file.

Since groups can be applied to most (soon [more](https://github.com/dbt-labs/dbt-core/issues/9339)) resource types, it would make sense to be able to define them in a root yml file.

See[ slack thread](https://dbt-labs.slack.com/archives/C05FWBP9X1U/p1730311796277499)

### Acceptance Criteria

I can define my group in a root-level yml file:

```yml
# /groups.yml

groups:
- name: marketing
owner:
name: marketing_pod
slack: ask-marketing-pod
```