Major code refactoring

Main goals are:
1. Ability to use mocks in unit tests instead of
having to setup mock web server as if it was an
integration test.
2. Cache Retrofit services in memory
3. Make it easier to read
4. Use OptIn where possible instead of propagating
Experimental* annotations everywhere

app/build.gradle

@@ -70,6 +70,12 @@ android {
     }
 }
 
+tasks.withType(org.jetbrains.kotlin.gradle.tasks.KotlinCompile).configureEach {
+    kotlinOptions {
+        freeCompilerArgs += "-opt-in=kotlin.RequiresOptIn"
+    }
+}
+
 dependencies {
     // https://github.com/material-components/material-components-android
     implementation "com.google.android.material:material:1.5.0"
@@ -110,7 +116,6 @@ dependencies {
     implementation platform("com.squareup.okhttp3:okhttp-bom:4.9.3")
     implementation "com.squareup.okhttp3:okhttp"
     debugImplementation "com.squareup.okhttp3:logging-interceptor"
-    testImplementation "com.squareup.okhttp3:mockwebserver"
 
     // https://github.com/Kotlin/kotlinx.serialization/releases
     implementation "org.jetbrains.kotlinx:kotlinx-serialization-json:1.3.2"
@@ -155,6 +160,9 @@ dependencies {
     // https://mvnrepository.com/artifact/com.google.truth/truth
     testImplementation "com.google.truth:truth:1.1.3"
 
+    // https://mockk.io/
+    testImplementation "io.mockk:mockk:1.12.3"
+
     // https://github.com/androidbroadcast/ViewBindingPropertyDelegate/releases
     implementation "com.github.kirich1409:viewbindingpropertydelegate-noreflection:1.5.6"