feat(cli): add display-name and availability to discriminated union fern definition

[RohinBhargava]

This PR introduces a new feature to the SingleUnionTypeSchema by adding a display-name field.

• The SingleUnionTypeSchema now includes the WithAvailabilitySchema import.
• The SingleUnionTypeSchema is extended with WithAvailabilitySchema.extend and WithNameAndDocsSchema.extend, which includes the display-name field.
• The SingleUnionTypeProperties now includes the displayName field.
• The SingleUnionType now extends commons.Declaration instead of commons.WithDocs.
• The SingleUnionType interface now includes the displayName field.
• The SingleUnionType serialization now includes the displayName field.

[dannysheridan]

Well-written PR description @billytrend-cohere!