Add rule to set primitive types to nullable

[wing328]

Add rule to set primitive types to nullable

e.g.

java -jar modules/openapi-generator-cli/target/openapi-generator-cli.jar generate -g java -i modules/openapi-generator/src/test/resources/3_0/petstore.yaml -o /tmp/java-okhttp/ --openapi-normalizer SET_PRIMITIVE_TYPES_TO_NULLABLE="integer|number"

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fyi @OpenAPITools/generator-core-team

[devhl-labs]

Why is a rule needed for this? Shouldn't it come from the spec?

[wing328]

ideally users should update the spec to indicate the schemas (e.g. string) are nullable but there are use cases in which users do not own the spec and as usual they don't want to maintain a modified version of the spec.