Feature/show activity list

[renefloor]

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Closes FLU-218
Closes FLU-219

related: GetStream/stream-core-flutter#3

CLA

• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

This is a big PR which adds a lot of boilerplate logic.

Sample app:

1. Sample app uses AutoRoute for navigation and has a guard for authentication.
2. For now the authentication (user_id) is stored in shared preferences and loaded on startup.
3. Sample app shows basic user feed when user is logged in.

Feeds SDK:
Feeds SDK is heavily inspired by the iOS SDK.

We have the FeedsClient that everybody has to use, it's connected to a userId and you have to call client.connect() to connect the websocket.

With client.feed(query) you create a feed object. By doing feed.getOrCreate() it will do an API call and you can start listening to updates.
The feed.stateNotifier is responsible for updating the state of the feed, for example using the websocket.
The FeedWsHandler is the one actually listening to websocket events and calling the stateNotifier to update when needed.
The stateNotifier.onReactionAdded and other similar methods can be used directly as well based on user interaction. Similar to the changeHandler on iOS.

Mapping between data and domain objects is done in the mappers file. Updating domain objects (using copyWith and immutable lists) is mostly done in the model classes directly.

To run this you'll need to have a local copy of the core sdk with code from this PR: GetStream/stream-core-flutter#3

Screenshots / Videos

flutter.feeds.demo.mov