Enhancement: Set isFreeFormObject property

[wheezil]

Description

A free-form object field in a model can be hard to distinguish from other types in the mustache template. Making this an explicit property would be helpful

openapi-generator version

3.3.1

OpenAPI declaration file content or url

	"object_field": { 
		"type":"object"
	}

Command line used for generation

This does not seem to be specific to any generator, although different generators deal with "object" differently -- some consider it to be a primitive type and others do not.

Steps to reproduce

Process any OpenAPI definition file with model containing a field like above.

Related issues/PRs

none

Suggest a fix/enhancement

For free-form object fields, set an isFreeFormObject property to true. Add this property field to CodegenParameter and CodegenProperty, as a peer to isString, isPrimitiveType, etc. I am not yet clear what logic should be used to set this field.