Unset a default value

[MikeEdgar]

As noted in smallrye/smallrye-open-api#2165 (comment), the @Schema annotation does not currently have any way to unset a default value or explicitly declare that none should be output in the OpenAPI. We should be able to support this with a constant value for the "UNSET" state in the @Schema

[Azquelt]

We need to think through the cases where you might want to unset the values:

• String
  • can have a reserved value
• Enum
  • could add a reserved value
• Another annotation
  • would need a special attribute on that annotation to state that the scanner should remove this value
• integer values
  • this one is tricky, I don't think there's a special value we can use in every case
• floating point values
  • May be able to use +/-Infinity or NaN in some cases

Any others?