Add return/scalar type declarations to the generated code

[SDParish]

The composer.json requires "php": ">=7.1". I am assuming this means that the generated php code is also targeting that version.

Since php 7, return type declarations are supported, along with scalar type declarations.
Since php 7.1, the void return type is supported as well.

I am proposing that the generated php classes are updated to provide these type declarations.

For example in a generated Request, the setBody function would gain the void return type, and sendWith would return SwaggerModel (assuming a response is not a simple scalar type), whilst in a generated Model, a string property's getter would have a string return type, and its setter have its argument declared as a string.