[BUG] [Spring] oneOf, anyOf support for List inside DTO

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
• [Optional] Sponsorship to speed up the bug fix or feature request (example)

Description

Spring openapi-generator-maven-plugin doesn't support oneOf.

openapi-generator version

4.3.1

pom.xml

 <plugin>
                <groupId>org.openapitools</groupId>
                <artifactId>openapi-generator-maven-plugin</artifactId>
                <version>4.3.1</version>
                <executions>
                    <execution>
                        <id>client</id>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <phase>generate-sources</phase>
                        <configuration>
                            <inputSpec>api.yml</inputSpec>
                            <output>codegen/client</output>
                            <generatorName>java</generatorName>
                            <configOptions>
                                <library>resttemplate</library>
                                <hideGenerationTimestamp>true</hideGenerationTimestamp>
                                <serializableModel>false</serializableModel>
                                <bigDecimalAsString>false</bigDecimalAsString>
                                <dateLibrary>java8</dateLibrary>
                                <java8>true</java8>
                                <interfaceOnly>true</interfaceOnly>
                            </configOptions>
                            <generateApiTests>false</generateApiTests>
                            <generateModelTests>false</generateModelTests>
                        </configuration>
                    </execution>
                    <execution>
                        <id>server</id>
                        <goals>
                            <goal>generate</goal>
                        </goals>
                        <phase>generate-sources</phase>
                        <configuration>
                            <inputSpec>api.yml</inputSpec>
                            <output>codegen/server</output>
                            <generatorName>spring</generatorName>
                            <configOptions>
                                <hideGenerationTimestamp>true</hideGenerationTimestamp>
                                <serializableModel>false</serializableModel>
                                <bigDecimalAsString>false</bigDecimalAsString>
                                <dateLibrary>java8</dateLibrary>
                                <java8>true</java8>
                                <interfaceOnly>true</interfaceOnly>
                                <useBeanValidation>true</useBeanValidation>
                            </configOptions>
                        </configuration>
                    </execution>
                </executions>
            </plugin>

OpenAPI declaration file content or url

I tried to generate:

api.yml

openapi: "3.0.0"
info:
  version: "1.0"
  title: "Main"

paths:
  /class:
    get:
      responses:
        200:
          description: Ok
          content:
            application/json:
              schema:
                $ref: "#/components/schemas/MainClass"

components:
  schemas:
    MainClass:
      type: object
      properties:
        fieldName:
          type: array
          items:
            oneOf:
              - $ref: "#/components/schemas/ObjectA"
              - $ref: "#/components/schemas/ObjectB"

    ObjectA:
      type: object
      properties:
        name:
          type: string
    ObjectB:
      type: object
      properties:
        name:
          type: string

Generation Details

When I use

...
<generatorName>java</generatorName>
   <configOptions>
        <library>resttemplate</library>
...

I get MainClass

...
public class MainClass {
  public static final String JSON_PROPERTY_FIELD_NAME = "fieldName";
  private List<MainClassFieldNameOneOf> fieldName = null;
...

which contains list of interfaces MainClassFieldNameOneOf. And ObjectA and ObjectB implement this interface.
Things are good.

BUT need SPRING and when use

...
<generatorName>spring</generatorName>
...

the interface OneOfObjectAObjectB is not generated

Steps to reproduce

• create a pom.xml with openapi-generator-maven-plugin 4.3.1
• create api.yml file above
• mvn clean generate-sources
• see broken code under target directory codegen\server\src\main\java\org\openapitools\model -> MainClass

Related issues/PRs

Suggest a fix

This bug is related to the Server side Java code (SpringBoot)
https://github.com/OpenAPITools/openapi-generator/issues/2906

Start with the supplied pom.xml and api.yml, then run mvn clean generate-sources to compare the Java and Spring outputs under target/codegen. Inspect the generated MainClass and model sources; done means the Spring generation includes the required oneOf interface and produces valid server code for the List field.