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/src/release/java/gq/kirmanak/mealient/di/ReleaseModule.kt

@@ -0,0 +1,20 @@
+package gq.kirmanak.mealient.di
+
+import dagger.Module
+import dagger.Provides
+import dagger.hilt.InstallIn
+import dagger.hilt.components.SingletonComponent
+import okhttp3.Interceptor
+import javax.inject.Singleton
+
+@Module
+@InstallIn(SingletonComponent::class)
+object ReleaseModule {
+
+  // Release version of the application doesn't have any interceptors but this Set
+  // is required by Dagger, so an empty Set is provided here
+  // Use @JvmSuppressWildcards because otherwise dagger can't inject it (https://stackoverflow.com/a/43149382)
+  @Provides
+  @Singleton
+  fun provideInterceptors(): Set<@JvmSuppressWildcards Interceptor> = emptySet()
+}