REQ-14868 - Support for custom template engine helpers

[yuval-po]

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Introduces support for use of custom template helpers via an option to provide a JAR path

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Overview

• Accept a new additionalProperties key templatingEngineHelper - a path to a JAR containing Mustache.Lambdas or Handlebars Helper<T> classes

• The template engine adapter enumerates the JAR for helpers for the currently selected engine

• Helpers are injected into the context on compile (Mustache) or registered on the engine (Handlebars)

• Helpers can be invoked in the template via their simple class name (e.g. {{#SomeHelper}}{{/SomeHelper}})

Rationale

• Provide support for more complex use-cases involving template logic not covered by built-in helpers

• Facilitate a way to 'inject' into the template context. Helper functions can serve as a bridge of sorts between OAG and user code. Some overlap with existing additionalProperties venue but allows for greater flexibility.

• Provide a way to mitigate certain issues without waiting for an official release or upgrading
  A real world example:

  • Descriptions may not be properly generated for some TypeScript models. This is solvable a template modification but due to what appears to be a bug in OAG the solution requires more logic than the built-in helpers provide.
    This forces the user to migrate to a fixed version, if available, incurring the usual effort costs or work-around the issue otherwise.
    This is especially useful in enterprise or API-proxy scenarios where it may be difficult to upgrade OAG version due to breaking schema changes

Caveats

• Helpers using imports not in the classpath will fail. While a more complete solution is most likely possible,
  I believe the feature has good value regardless.

• Includes unit tests
• Includes code documentation
• Compiled project and tested via openapi-generator-cli
• Squash required

Closes #14868

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