Stats function implementation

[ikurek]

This is a rework of #283 because the previous PR was made from enhancement/formatting and the branch can't be merged. The logic had to be extended, since the solution presented in previous PR didn't work.

Original description from #283:

This PR includes full implementation of Rest.stats function for Flutter plugin. As stats is a slightly complicated and big object, this pull request might seem a bit huge but the scope is still just a map of a function.
Implemenation contains

• New type definitions on Flutter side for code generation
• New encoders for Java
• New encoders for Swift/Objective-C
• Function mappings
• A paginated result viewer addition to example app

There were some differences between native implementations:

• Structures were formed slightly differently. I created an issue in ably-java for this
• Stats structure is not fully compatible with spec ably-java#733

The way stats params were received was a bit more structured in Objective C than Java. In Objective C there is a dedicated object for this, where it's a Param[] in Java. I have created a separate category in Objective C to build that object from the map sent from Dart side.

Additions from my side:

• Removed unnecessary formatting and changes
• Fixed issue with null handle being passed to iOS method
• Implemented dart decoder for Stats objects

This pull request remains blocked untill ably-java#733 and ably-cocoa#1284 are resolved, so I'm leaving it as a draft