Feat: Add support for dot env file to load variables from

[themisvaltinos]

This update allows environment variables being loaded from a .env file in the project directory, fixes: #4405

Example .env file:

SNOWFLAKE_PW=my_secret_password
S3_BUCKET=s3://my-data-bucket/warehouse

# Override SQLMesh config values
SQLMESH__DEFAULT_GATEWAY=production
SQLMESH__MODEL_DEFAULTS__DIALECT=snowflake

[georgesittas]

@themisvaltinos we need to move this to dependencies– this isn't a dev dependency, because the .env functionality requires that this package is installed on the user's system.