Optional transformer argument for parameter types

[kieran-ryan]

🤔 What's changed?

• Made the Cucumber Expressions parameter type optional

⚡️ What's your motivation?

• The transformer is intended to be optional - casting to the type by default if not explicitly specified (see fix: ParameterType constructor cucumber/cucumber-expressions#178). Python support available through python: optional transformer argument for parameter types cucumber/cucumber-expressions#288.
• This improves user experience, removing a common use case to specify casting the value to a callable (the type) without any further transformation. This is in turn improves code quality and maintainability.

🏷️ What kind of change is this?

• ⚡ New feature (non-breaking change which adds new behaviour)