feat: Add support for feed subscription

[xsahil03x]

Fixes: #55, Porting our Dart implementation in Java
https://github.com/GetStream/stream-feed-flutter

A sample usage

// APPID IS REQUIRED IN ORDER TO SUBSCRIBE FEEDS
CloudClient cloudClient = CloudClient.builder(apiKey, token, "chris", appId).build();

CloudFlatFeed chris = cloudClient.flatFeed("user", "chris");

// PRINTS ANY CHANGES IN THE FEED
var subscription = chris.subscribe(message -> System.out.println(message.toString())).get();

chris.addActivity(
      Activity.builder()
              .actor("SU:chris")
              .verb("tweet")
              .object("tweet:10")
              .extraField("message", "Beautiful bird!")
              .build()
     ).get();
     
subscription.cancel();

[xsahil03x]

@ferhatelmas I have a different formatting configuration in my IntelliJ, so can you also maybe reformat all and push.

[ferhatelmas]

can you also maybe reformat all and push.

@xsahil03x don't worry about it, will update CI to do it automatically.

[ferhatelmas]

@peterdeme let's review and try to release this

[peterdeme]

I don't think this logic will work how you imagined. You need to swap the conditions.

[peterdeme]

@xsahil03x looks good to me except for my small comment. Can you add tests too?

[peterdeme]

@xsahil03x @ferhatelmas I copied the contents of this PR and opened a new one: #110

Please follow that one. @xsahil03x - thanks so much for your contribution 🙏 we'll try to release it ASAP.