Refactor how requests to CloudRouter are made

[shawnmaten]

The requests to CloudRouter (the internal service that handles file listing, transfers, etc for cloud storage) are currently made using Retrofit and involve POJO serialization. But the service isn't RESTful, it's really a JSON-RPC, and Retrofit doesn't integrate well with it. And because of its response formatting, it also doesn't work well with POJO serialization.

I think it would be less clunky if we either 1) found a better library to use for this or 2) did something more basic with OkHttp and Gson. I didn't really like this when I made it, but now there's cause to fix it because the awkwardness of trying to map everything to a Java object is partly what lead to an issue in the Android SDK. I mapped the S3 headers to an object, but it turns out I didn't know all the headers to expect.

More formally fixes the below:

• Signature does not match error filestack-android#140
• Send AWS S3 encryption header #52

[shawnmaten]

Moved to internal tracker: https://filestack.atlassian.net/browse/FS-3349