Make "applyToParams" a Future + add "this." when decoding JSON

Created by: noordawod

This pr brings back 2 lost "features" that were present earlier (and of course with the proper explanation as to why we need them back), and introduces a couple of new features as well:

1. Inside a function, private-style variables (var _json) make no sense – so removed the underscore.
2. The authNames were mysteriously dropped, why? These are important information to each endpoint, and the authentication implementation may need to check which authentications to apply for each operation.
3. Removed use of the bang operator (!) when activating the authentication implementation – just use what Dart gives us out of the box: authentication?.applyToParams(…)
4. Converted the applyToParams method to a Future<void>. Custom implementations may need to retrieve data from persistent stores, for example, and so may need this.
5. Since Authentication is an interface, and in case ApiClient is inherited, one can now declare the generic T extends Authentication class upon extension. Helps to access properties added in the inherited class (yes, this is backward-compatible and Dart, luckily, doesn't scream about it.)

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