Implement an ApiFile helper for verifying signatures and uploading them, as well as deltas.

Also, identify when deltas are required by the API as well.

A weird yet neat way to implement this is to make extension libraries that build onto the VRChat builder, like so `builder.WithSignatures()` and `builder.WithAmazonUploading()` to instantiate those two helpers, which the extension libraries will implement, say, an `IVRChatS3Uploader` and an `IVRChatSignatureBroker`. These will be used in ApiFile to instantiate and perform those tasks if needed. `VRChat.Signatures` and `VRChat.S3` packages could be downloaded to implement these.