Add proguard rules for Retrofit + suspend functions

Context:
square/retrofit#3751 (comment)

app/proguard-rules.pro

@@ -13,6 +13,15 @@
 -dontwarn org.conscrypt.**
 -dontwarn org.bouncycastle.**
 -dontwarn org.openjsse.**
+
+# https://github.com/square/retrofit/issues/3751#issuecomment-1192043644
+# Keep generic signature of Call, Response (R8 full mode strips signatures from non-kept items).
+-keep,allowobfuscation,allowshrinking interface retrofit2.Call
+-keep,allowobfuscation,allowshrinking class retrofit2.Response
+# With R8 full mode generic signatures are stripped for classes that are not
+# kept. Suspend functions are wrapped in continuations where the type argument
+# is used.
+-keep,allowobfuscation,allowshrinking class kotlin.coroutines.Continuation
 # endregion
 
 # region okio - https://square.github.io/okio/#r8-proguard