Use dedicated DocStringTypeRegistry for doc string transforms

[mpkorstanje]

Summary

Cucumber uses the DatatTableTypeRegistry to convert doc strings. This is rather confusing and ignores the type provided with the doc string.

Motivation & Context

With #1458 and #851 implemented it will be quite hard to explain how doc strings are converted. So it would be better to do it properly.

Current Behavior

Given some gherkin

Given a doc string
"""json
{hello:"world"}
"""

And a step definition:

@Given("a doc string")
public void a_doc_string(JsonNode json){

}

In the StepExpressionFactory Cucumber will try to transform this string into JSON by turning the docstring into a 1x1 table and using the DataTableTypeRegistry to convert it to an object of the right target type.

DocStringTransformer<?> docStringTransform = new DocStringTransformer<Object>() {
    @Override
    public Object transform(String docString) {
        Type targetType = tableOrDocStringType.resolve();
        if (Object.class.equals(targetType)) {
            return docString;
        }

        List<List<String>> raw = singletonList(singletonList(docString));
        return DataTable.create(raw, StepExpressionFactory.this.tableConverter).convert(targetType, transpose);
    }
};

Expected Behavior

Cucumber uses a DocStringTypeRegistry in which transformations for each document type can be registered or where a default doc string transformer (document, type) -> Object can be registered.