[gradle]: allow setting custom engines other than handlebars

[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: #5686

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[wing328]

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

Thanks for the PR. What other values are allowed?

Shall we validate the input to only allow handlebars, mustache ?

[PhilippHeuer]

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

Thanks for the PR. What other values are allowed?

Shall we validate the input to only allow handlebars, mustache ?

When working with a custom engine (https://openapi-generator.tech/docs/templating/#custom-engines) - example https://github.com/twitch4j/nitro-codegen/blob/master/template-engine/src/main/java/com/github/twitch4j/codegen/engine/PebbleEngineAdapter.java#L26 - it's important that users can provide the custom identifier value (peb in the example), otherwise it's not possible to use custom template engine adapters with the gradle plugin.
Therefore any value can be valid, this depends on the identifiers used in custom template engine adapters - this makes value validation hard.

If users provide a invalid value than the openapi-generator will return this kind of error:

Exception in thread "main" java.lang.RuntimeException: Couldn't load template engine adapter my-custom-engine. Available options:
mustache
handlebars

Therefore i don't think we need this kind of validation in the gradle plugin, as we already get good error messages from the generator.

[wing328]

@PhilippHeuer thanks for the PR and the explanation. Makes sense to me.