Specify required relationship fields in OpenAPI schema

[arttuperala]

Description of the Change

Required relationships are added to the required array under relationships in the OpenAPI schema.

    post:
      operationId: create/entries
      requestBody:
        content:
          application/vnd.api+json:
            schema:
              required: ...
              properties:
                data:
                  type: object
                  required: ...
                  properties:
                    type: ...
                    id: ...
                    links: ....
                    attributes: ...
                    relationships:
                      type: object
                      properties:
                        blog:
                          $ref: '#/components/schemas/reltoone'
                        blogHyperlinked:
                          $ref: '#/components/schemas/reltoone'
                        authors:
                          $ref: '#/components/schemas/reltomany'
                        comments:
                          $ref: '#/components/schemas/reltomany'
                        commentsHyperlinked:
                          $ref: '#/components/schemas/reltomany'
                        suggested:
                          $ref: '#/components/schemas/reltomany'
                        suggestedHyperlinked:
                          $ref: '#/components/schemas/reltomany'
                        tags:
                          $ref: '#/components/schemas/reltomany'
                        featuredHyperlinked:
                          $ref: '#/components/schemas/reltoone'
                      required:
                      - blog
                      - authors

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[sliverc]

Thanks. Looks good.