Explanation

The PR contains data module, necessary libraries for API calls and Topic Management System models, retrofit instance and interceptor.

Also this PR was earlier on #61 but because of merge conflicts I have introduced this new PR.

Libraries Used

1. Retrofit: implementation 'com.squareup.retrofit2:retrofit:2.5.0'
   This library is responsible for making API calls to Oppia backend
2. Retrofit Moshi Converter: implementation 'com.squareup.retrofit2:converter-moshi:2.5.0'
   This library is the actual deserializer that we are using over GSON.
3. Moshi Kotlin Support: implementation"com.squareup.moshi:moshi-kotlin:1.8.0"
   Moshi Kotlin support library for using Moshi in Kotlin.

Issues

The models in this PR does not have custom field names because @JSON in Moshi which is similar to @SerializedName in GSON is not working with Kotlin Moshi.

Checklist

Work items:

• Identify among all existing GAE endpoints (get/post), which we can use during the prototype
• Loosely identify data missing from the above endpoints (this bucket is expected to be small/non-existent), and identify which endpoints have data which needs to be changed for the Android app
• Specify the Retrofit interfaces to represent all of the identified endpoints (simple interfaces/no annotations or return types needed beyond pseudocode)
• Identify** what data (via pseudocode data structures/YAML/JSON/etc) we will be passing along from the backend through these endpoints
• Identify how error handling will work (e.g. what Retrofit does), whether RPC retry is supported & how it is/can be configured, etc.
• Determine the testing strategy for the GAE endpoints for downstream app components