[gradle]: allow setting custom engines other than handlebars

Created by: PhilippHeuer

The openapi-gradle generator plugin has a strict check on the engine param to only set the parameter if handlebars is set as value. Otherwise the parameter is ignored. This is currently blocking anyone from specifiying custom template engines (https://openapi-generator.tech/docs/templating/#custom-engines) when using the official gradle plugin.

This change will set the engine param to the user-provided value, even if it's not handlebars.

The engine param was initially added in: https://github.com/OpenAPITools/openapi-generator/pull/5686

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