Add r8/proguard configuration to keep GSON related classes. (#1348)

Specifically keep data class members used in retrofit responses.

Corona-Warn-App/proguard-rules.pro

@@ -57,4 +57,34 @@
 # With R8 full mode, it sees no subtypes of Retrofit interfaces since they are created with a Proxy
 # and replaces all potential values with null. Explicitly keeping the interfaces prevents this.
 -if interface * { @retrofit2.http.* <methods>; }
--keep,allowobfuscation interface <1>
\ No newline at end of file
+-keep,allowobfuscation interface <1>
+
+
+##---------------Begin: proguard configuration for Gson  ----------
+# Gson uses generic type information stored in a class file when working with fields. Proguard
+# removes such information by default, so configure it to keep all of it.
+-keepattributes Signature
+
+# For using GSON @Expose annotation
+-keepattributes *Annotation*
+
+# Gson specific classes
+-dontwarn sun.misc.**
+#-keep class com.google.gson.stream.** { *; }
+
+# Application classes that will be serialized/deserialized over Gson
+-keep class com.google.gson.examples.android.model.** { <fields>; }
+
+# Prevent proguard from stripping interface information from TypeAdapter, TypeAdapterFactory,
+# JsonSerializer, JsonDeserializer instances (so they can be used in @JsonAdapter)
+-keep class * extends com.google.gson.TypeAdapter
+-keep class * implements com.google.gson.TypeAdapterFactory
+-keep class * implements com.google.gson.JsonSerializer
+-keep class * implements com.google.gson.JsonDeserializer
+
+# Prevent R8 from leaving Data object members always null
+-keepclassmembers,allowobfuscation class * {
+  @com.google.gson.annotations.SerializedName <fields>;
+}
+
+##---------------End: proguard configuration for Gson  ----------
\ No newline at end of file