feat(spec): allow additional fields on enum refs

[Algram]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: N/A

What is the new behavior?

If there is an enumName supplied, all the additionalFields are currently just added as a ref-sibling. This is not swagger-spec compliant and therefore gets ignored.

Does this PR introduce a breaking change?

• Yes
• No

Other information

See this issue for more information: #2806

[Algram]

I was not sure if we should just allow all fields, so I added an allowlist here. I did not find any information in the swagger spec on what they technically support on an enum ref field.

[kamilmysliwiec]

LGTM

[0816i]

Is there any reason to assume metadata of ApiProperty as enum's metadata? I have problem by this pr.

I declared dto as below.

enum TestEnum {
    A = "A",
    B = "B"
}

class EnumTest {
  @ApiProperty({
    enum: TestEnum,
    enumName: 'TestEnum',
    isArray: true,
    default: [],
  })
  readonly testEnum: TestEnum[] = [];
}

A class called EnumTest has a property called testEnum, which expects TestEnum to enter the form of an array.
If so, I want the yaml as below.

components:
  schemas:
    TestEnum:
      type: string
      enum:
        - A
        - B
# .....
    EnumTest
      type: object
      properties:
        testEnum:
          type: array
          default: []
          items:
            $ref: "#/components/schemas/TestEnum"

But I got the yaml as below.

components:
  schemas:
    TestEnum:
      type: string
      default: [] # It cause error on openAPI specification.
      enum:
        - A
        - B
# .....
    EnumTest
      type: object
      properties:
        testEnum:
          type: array
          items:
            $ref: "#/components/schemas/TestEnum"

I know that ['description', 'deprecated', 'default'] can be declared as a field of specific properties. I think that It does not seem appropriate to omit these three fields on EnumTest definition and declare metadata of EnumTest's property on TestEnum's schema.

[0816i]

In my opinion, if you want to field description and default for enum, it looks good to create another field that is isolated from property.