[REQ] Better collection serialization support

[ackintosh]

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

Since OAS 3.0, in order to support common ways of serializing simple parameters, style and explode are defined instead of collectionFormat.

On the other hand, we still using collectionFormat: style and explode parameters are mapped to collectionFormat.

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

Lines 3083 to 3094 in 7dbda04

	// TDOO revise collectionFormat
	String collectionFormat = null;
	if (ModelUtils.isArraySchema(parameterSchema)) { // for array parameter
	final ArraySchema arraySchema = (ArraySchema) parameterSchema;
	Schema inner = getSchemaItems(arraySchema);
	if (arraySchema.getItems() == null) {
	arraySchema.setItems(inner);
	}
	collectionFormat = getCollectionFormat(parameter);
	// default to csv:
	collectionFormat = StringUtils.isEmpty(collectionFormat) ? "csv" : collectionFormat;

openapi-generator/modules/openapi-generator/src/main/java/org/openapitools/codegen/DefaultCodegen.java

Lines 4592 to 4609 in 7dbda04

	protected String getCollectionFormat(Parameter parameter) {
	if (Parameter.StyleEnum.FORM.equals(parameter.getStyle())) {
	// Ref: https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.1.md#style-values
	if (Boolean.TRUE.equals(parameter.getExplode())) { // explode is true (default)
	return "multi";
	} else {
	return "csv";
	}
	} else if (Parameter.StyleEnum.SIMPLE.equals(parameter.getStyle())) {
	return "csv";
	} else if (Parameter.StyleEnum.PIPEDELIMITED.equals(parameter.getStyle())) {
	return "pipe";
	} else if (Parameter.StyleEnum.SPACEDELIMITED.equals(parameter.getStyle())) {
	return "space";
	} else {
	return null;
	}
	}

This can not representate serialization format required in OAS3.

Describe the solution you'd like

In order to make available the serialization format, explode and style parameter values should be assigned to mustache templates, and handle the parameters in each generator.

We need to keep collectionFormat until generators support style and explode.

Additional context

Related PR: #3984

[ybelenko]

Can we close this in favor of #4042 ?