[Feature] Adding local variables in the models that can be used in tests, model objects, etc

[moattarwork]

Is this your first time submitting a feature request?

• I have read the expectations for open source contributors
• I have searched the existing issues, and I could not find an existing issue for this feature
• I am requesting a straightforward extension of existing dbt functionality, rather than a Big Idea better suited to a discussion

Describe the feature

Currently, project file variables can be used to share values between the tests and model objects, and the feature provides a way to pass the values from the command line. However, in most scenarios, the requirement is the local variables, which can still be provided in this method. However, it might cause a large list of variables in the project file.
The solution might be a local variable that can be easily shared in the scope of each model file and all the model-related objects, such as tests, data tests, and SQL objects. The same concept is available in well-known IAC frameworks such as Terraform.

Describe alternatives you've considered

No response

Who will this benefit?

No response

Are you interested in contributing this feature?

No response

Anything else?

No response