feat: transform union_distinct op into set operation that supports all enumerations

[dshaaban01]

• Changed UnionDistinctOp into SetOp that contains enum's for all the set relations. Adjusted valid and invalid test cases to include all the enumerations.
• Adjusted return type inference function to handle all enum's.
• Adjusted export function to handle other enum's + adjusted test case to handle all enumerations.
• Adjusted import function to handle other enum's + adjusted test case to handle all enumerations.

Note: This implementation only works for two inputs. TODO: extend to support repeatable inputs.

[dshaaban01]

@ingomueller-net

Enums ready!

[CLAassistant]

All committers have signed the CLA.