[REQ] Support for registering additional handlebars helpers
Created by: childish-sambino
Is your feature request related to a problem? Please describe.
I'm unable to register additional helpers in the HandlebarsEngineAdapter: https://github.com/OpenAPITools/openapi-generator/blob/057a56a8272aa9723278acc860abd6b24890d582/modules/openapi-generator/src/main/java/org/openapitools/codegen/templating/HandlebarsEngineAdapter.java#L80-L88
Describe the solution you'd like
Support for registering a callback function that is invoked with the Handlebars instance, or refactoring HandlebarsEngineAdapter so that it may be extended to without duplicating all the compileTemplate logic.
Describe alternatives you've considered
Currently I've extended HandlebarsEngineAdapter, but also had to copy most logic from compileTemplate as it's not extensible.