OpenAPI Generator: Expose configuration for import mapping / schema mapping

[shataya]

Feature description

Hi,

OpenAPITools support customization by schema mapping and import mapping:

See: https://openapi-generator.tech/docs/customization/#schema-mapping
and: https://openapi-generator.tech/docs/customization/#bringing-your-own-models

It seems that these two options are already used internally, but are not made available for external configuration (especially for the gradle plugin).

It would be very helpful to provide these two options to reuse models from your own libraries instead of having them generated. For example, you can use existing models for error responses.