Add `column_schemas` attribute

[tamargrey]

• As a user, I wish I could access a table's column schema with a column_schemas attribute that is a dictionary of column schemas.

df.ww.column_schemas

This could be useful for helping users understand that they can df.ww.column_schemas[col] instead of df.ww[col].schema better than the columns attribute does.

We should not remove the columns attribute so we don't add a breaking change.