Refactor(optimizer): factor out pseudocolumns in qualify columns

[georgesittas]

Checking if dialect == "bigquery" in qualify_columns isn't very elegant. This PR abstracts that section away by introducing a Dialect attribute for storing pseudo-column names.

There are some other dialects that have pseudo-columns besides BigQuery, such as:

• Redshift: https://docs.aws.amazon.com/redshift/latest/dg/c-spectrum-external-tables.html#c-spectrum-external-tables-pseudocolumns
• Databricks: https://docs.databricks.com/ingestion/file-metadata-column.html
• Snowflake: https://docs.snowflake.com/en/sql-reference/sql/create-external-table#optional-parameters

This attribute will make it easy to control what's expanded in SELECT * queries for them too, if ever needed.

cc: @z3z1ma