Deps: Add retrofit transitively used dependencies on wordpress

It is generally recommended that transitively used dependencies should
be declared directly.

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Actually, updating 'Zendesk', which was the dependency that was
providing this transitively used 'Retrofit' dependency to the project,
was causing a compilation failure. This is due to the fact that
the [TenorGifClient] is directly using this dependency, but could no
longer resolve it.

PS: This is (most probably) due to the fact that this 'Retrofit'
dependency was no longer available on newer 'Zendesk' versions.

WordPress/build.gradle

@@ -413,6 +413,7 @@ dependencies {
     implementation "org.greenrobot:eventbus:$eventBusVersion"
     implementation "org.greenrobot:eventbus-java:$eventBusVersion"
     implementation "com.squareup.okio:okio:$squareupOkioVersion"
+    implementation "com.squareup.retrofit2:retrofit:$squareupRetrofitVersion"
     implementation "org.apache.commons:commons-text:$apacheCommonsTextVersion"
     implementation "com.airbnb.android:lottie:$lottieVersion"
     implementation "com.airbnb.android:lottie-compose:$lottieVersion"

build.gradle

@@ -75,6 +75,7 @@ ext {
     philjayMpAndroidChartVersion = 'v3.1.0'
     squareupKotlinPoetVersion = '1.6.0'
     squareupOkioVersion = '2.8.0'
+    squareupRetrofitVersion = '2.9.0'
     uCropVersion = '2.2.4'
     zendeskVersion = '5.0.2'