Fix duplicated operationId with new rule in normalizer

[wing328]

Fix duplicated operationId (within the same tags) with new rule in normalizer:

• FIX_DUPLICATED_OPERATIONID: When set to true, an integer suffix will be added to duplicated operationId(s), e.g. getName => getName_0, getName_1, etc

Example:

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g java -i modules/openapi-generator/src/test/resources/3_1/java/petstore.yaml -o /tmp/java-okhttp/ --openapi-normalizer FIX_DUPLICATED_OPERATIONID=true

One can use the operationIdNameMappings option to further custom the name: https://github.com/openapitools/openapi-generator/blob/master/docs/customization.md#name-mapping

fyi @OpenAPITools/generator-core-team

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