[IBIS] OmniSciDB DDL/Client Refactoring

[xmnlab]

OmniSciDB DDL/Client Refactoring

[saulshanabrook]

• drop(columns) in ibis should be mapped to ALTER TABLE .... DROP in omnisci
• add(columns) should be mapped to ALTER TABLE .... ADD in omnisci
• assign(columns) should be mapped to ALTER TABLE + UPDATE in omnisci.

[xmnlab]

1. add_column/drop_column PR opened by @YarShev https://github.com/ibis-project/ibis/pull/2094
2. https://github.com/ibis-project/ibis/pull/2086 refactor OmniSciDB DDL module and add rename_table, rename_columns and rename_user methods. It also fixes some bugs

[xmnlab]

refactoring merged.