make parts of GraphQLGetTransformer and MultipartFormData public

[ethansinjin]

This allows creating NetworkTransport subclasses outside of the Apollo module that still make use of GraphQLGETTransformer and MultipartFormData.
A use case for doing this (and my actual use case) is replacing the use of URLSessionTask in HTTPNetworkTransport.swift with another networking library, such as Alamofire (ex. AlamofireNetworkTransport.swift)

[designatednerd]

Hi! A couple questions:

• What's your reason for wanting to subclass rather than making your own implementation of the NetworkTransport protocol?
• Wouldn't we need to make HTTPNetworkTransport have an open annotation for this to work? Or is that implicit since it's not final?

[ethansinjin]

Hey, thanks for the response! Sorry, I misspoke in my original message. I am indeed implementing the NetworkTransport protocol, not subclassing HTTPNetworkTransport. However, my class is effectively mirroring HTTPNetworkTransport, and only changing parts related to creating a URLSessionClient, URLSessionTask, and URLRequest. Because only those parts are changing, it would be helpful to use GraphQLGETTransformer and MultipartFormData in the same way that HTTPNetworkTransport's implementation does, to avoid having to reimplement those classes as well. At the moment, that can't be done due to the lack of public annotations on the classes themselves, as well as some of their properties. That's what this PR aims to resolve.

[designatednerd]

Got it, thank you for the clarification. I think these are fine to make public. Thanks for the PR!