Added retrofit data classes to proguard keep rules (#214)

Corona-Warn-App/proguard-rules.pro

@@ -58,3 +58,6 @@
 # and replaces all potential values with null. Explicitly keeping the interfaces prevents this.
 -if interface * { @retrofit2.http.* <methods>; }
 -keep,allowobfuscation interface <1>
+
+-keep class de.rki.coronawarnapp.http.requests.* { *; }
+-keep class de.rki.coronawarnapp.http.responses.* { *; }
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