feat: allow passing variables that may exist in the loaded SQL files

[lancetipton]

• Allows passing in a dict containing key-value pairs of variables
• Does a simple replace call on each key, and replaces it with the value

Example

Original SQL

select zip, street from ${schema}_places.${type}_address;

Lineagex Call

    variables={
      '${schema}': 'staging',
      '${type}': 'home',
    }

    lineagex(sql='/path/to/sql/files', conn_string='...', variables=variables)

SQL after variable replacement

select zip, street from staging_places.home_address;