[Feature] Set vars via environment variables

### 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

Enable to set the vars option via environment variables, `DBT_VARS`, like `DBT_PROFILE`, `DBT_TARGET`

### Describe alternatives you've considered

_No response_

### Who will this benefit?

Who have tools execute dbt commands. In my case, I have a wrapper script for dbt cli and IDE extension can execute the commands, and want to have more flexibility.

### Are you interested in contributing this feature?

Yes, if the idea looks good

### Anything else?

_No response_