ultimate path template generation

[alexander-fenster]

Let's generate all path templates that are either 1. or 2. or both:

1. Directly referenced from a message with google.api.resource_reference [this functionality already exists]

2. If a resource is defined for some message (e.g. message Book) and there exists another message that directly uses that Book message (e.g. message UpdateBookRequest has Book book = 1), we consider this resource to be used and generate path templates for it. [this should be added!]

[alexander-fenster]

Will be taken care of by #211.