[REQ] [SPRING] Add support for importing custom lambdas

Is your feature request related to a problem? Please describe.

I would like to be able to use a lambda for converting text from camelCase to SCREAMING_SNAKE_CASE using Spring codegen.

Describe the solution you'd like

A custom lambda directory defined using configuration.lambdaDirectory would be scanned for custom mustache lambdas, that would implement Mustache.Lambda interface. These would be registered with the rest of Spring lambdas, so they could be used from within custom mustache templates.

Describe alternatives you've considered

I considered adding this as a new lambda in Spring codegen, however there is a lot of combinations of case to case conversions, which can be implemented using different libraries, so a custom solution is probably better.

Additional context

https://github.com/OpenAPITools/openapi-generator/blob/master/docs/templating.md#mustache-lambdas

Start with the templating documentation section on Mustache lambdas and the Spring codegen entry points that register existing lambdas. Trace how configuration.lambdaDirectory could be discovered and custom Mustache.Lambda implementations registered; done means a custom lambda can be used from a Spring codegen template.