Upgraded RxJava from version 2 to 3

[Akash-Ramjyothi]

Fixes: #918.

Description:

Updated all the dependencies and usage of RxJava from version 2 to 3 and reactive types (First Revision).

[SebaDro]

Hey @Akash-Ramjyothi . Please, change your pull request so that it will be merged into the develop branch. Things have to be tested before they will be merged into the master branch. In addition, make sure that everything is working fine after the library upgrade. This also includes automate Gradle builds via GitHub Actions.

[asaikarthikeya]

Hello @SebaDro. This PR is a part of our GSoC 2022 efforts, Is it okay to directly direct the changes to develop branch? Or create a separate branch named gsoc2022 (similar to last year) and later merge it into develop once gsoc ends?

[SebaDro]

Upgrading RxJava from 2 to 3 may come with some braking changes, so many things have to be tested afterwards. Therefore, I'd suggest to do all changes and implementations as part of a separate GSoC branch.