Add Retrofit and MockWebServer libraries

build.gradle

dependency {
    def retrofitVersion = '2.6.1'
    def mockwebserverVersion = '3.8.1'
    implementation "com.squareup.retrofit2:retrofit:$retrofitVersion"
    implementation "com.squareup.retrofit2:converter-gson:$retrofitVersion"
    implementation "com.squareup.okhttp3:mockwebserver:$mockwebserverVersion"
}