Share common attributes of extensions in the same class

When using all three extensions porvided by this plugin, configuration often looks like:
```
openapi {
title = project.name
description = project.description.toString()
version = project.version.toString()
format = "yaml"
}

openapi3 {
title = project.name
description = project.description.toString()
version = project.version.toString()
format = "yaml"
}

postman {
title = project.name
description = project.description.toString()
version = project.version.toString()
}
```

As `openapi` and `openapi3` share the same base class, above code could be factorized:
```
configure( listOf( openapi, openapi3 )) {
title = project.name
description = project.description.toString()
version = project.version.toString()
format = "yaml"
}
```
But unfortunately it cannot be applied to `postman` extension.

It should be useful that all those three extensions share their common attributes in the same class, in order to ease the configuration step.