Stats structure is not fully compatible with spec

[ikbalkaya]

While working on implementing stats for Flutter SDK
I realised that ably-java and ably-cocoa have slightly different public stats interfaces.

• Ably-java has a processed field which is a type of ProcessedMessages. I couldn't find a mapping here in spec
• MessageType contains fields of MessageCategory that is a subclass of MessageCount and MessageCategory. Ruby docs specifies that those fields should be simply MessageCount fields. MessageCount also contains an additional uncompressedData which wasn't on spec.

┆Issue is synchronized with this Jira Story by Unito