Add Generator as a subspec

[ryu1sazae]

• Add a subspec to be able to use generator under a /Generator/* directory.

Originally, we couldn't use the generator when we download through Pod cuz source files is specified like below. https://github.com/uber/needle/blob/d3eaf696ad2e4c3e25618cfa3643402fdd237f7a/NeedleFoundation.podspec#L12

That's why, I wanna make it possible to use the generator through Pod by adding a subspec. And I suppose usage is like this. pod 'NeedleFoundation/Generator'

And I think adding another podspec file is gonna be also fine other than NeedleFoundation.podspec. But in terms of a maintenance cost, I took the way of adding a subspec.