[NO-TICKET] Enable default value support for union types

[timdawborn]

Description, Motivation and Context

Support for the @default annotation does already exist in SPOT, but only for certain types. This PR enables the @default annotation for union types, allowing default values for enums.

Checklist:

• [ ] I've added/updated tests to cover my changes
• [ ] I've created an issue associated with this PR

[CLAassistant]

All committers have signed the CLA.