Add extra StepDefinitionPatternType for other than original gherkin realizations

### 🤔 What's the problem you're trying to solve?

As a developer of https://github.com/elchupanebrej/pytest-bdd-ng I try to implement messages protocol. pytest-bdd-ng have extra step definition patterns supported, so it can't fire correct messages

### ✨ What's your proposed solution?
Add extra type to define non-original step definition patterns; Make it non-required

https://github.com/cucumber/messages/blob/5b02bdc9b94d548f96b22950f1dfe8bc8f06db6e/jsonschema/StepDefinition.json#L18

Start with jsonschema/StepDefinition.json at the StepDefinitionPatternType reference around line 18. Review the existing type definition and determine how an optional value for non-original Gherkin step-definition patterns should be represented; done means the schema accepts that additional pattern type without requiring it.

Written by the indexing model from the issue text.