Implemented ApiService Class

[chandansgowda]

This PR includes the following changes:

1. Created a http client singleton instance with the auth headers, interceptors which can be used for all calls
2. Used Chopper Generator to generate http client
3. Added an ApiService class with methods to CREATE, JOIN and DELETE rooms (Sends requests to Resonate Backend API) - Closes Implement ApiService Class #68
4. Also, Documentation has been updated to reflect recent project migration to Appwrite - Closes Migrate Backend from Firebase to Appwrite #63

The previous PR for the same was closed since the branch had to be renamed for good documentation of the project history. Considering @jddeep review on my previous PR, work on points 1 and 2 above is done.