[REQ] Mustache lambda for pretty-printing JSON structures

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

I'd like to be able to pretty-print JSON structures with the use of a Mustache lambda. My use case is that I'd like my custom templates to show examples that were embedded within their originating OpenAPI specs. However, JSON objects and arrays are currently printed compactly, which isn't easy to read.

Describe the solution you'd like

A lambda that looks something like this:

{{#lambda.pretty}}{{someVariable}}{{/lambda.pretty}}

And it would work like this:

• Booleans, numbers, strings, and null are printed as-is.
• Objects and arrays are pretty-printed; their constituent elements are indented and printed on multiple lines.

Describe alternatives you've considered

Grinning and bearing it. Since some of the libraries used in this project already offer JSON pretty-printing, I think implementing it as a standard lambda would be the past of least resistance.

Start with the OpenAPI Generator templating and Mustache lambda documentation linked in the issue, then locate the existing standard lambda implementations. Add a pretty-printing lambda with the requested scalar, object, and array behavior, and verify it through the relevant templating tests or examples.