[CT-3563] [source] configuration of "database" in dbt_project.yml or source.yml

### Is this your first time submitting a feature request?

- [X] I have read the [expectations for open source contributors](https://docs.getdbt.com/docs/contributing/oss-expectations)
- [X] I have searched the existing issues, and I could not find an existing issue for this feature
- [X] I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

### Describe the feature

I want to configure the database for all sources in the "models" directory.

This works:

```
-- sources.yml
sources:
my_source:
+database: my_database_prod

```

This doesn't:

```
-- dbt_project.yml
sources:
my_dbt_project:
models:
+database: my_database_prod

```
error:
```
[WARNING] Configuration paths exist in you dbt_pproject.yml which do not apply to resources.
There are 1 unused configuration paths:
- sources.my_dbt_project.models

```

### Describe alternatives you've considered

configuring all source files individually.

### Who will this benefit?

Users who:
- use separate databases for separate target environments.
- want to save $ but not copying source data from prod to other environments OR
- do not have permission to copy source data from prod to other environments

### Are you interested in contributing this feature?

if it's easy and I can understand how to test it

### Anything else?

Testing was the hard part last time I tried to contribute