fix: add Retrofit R8 rules

android/proguard-rules.pro

@@ -1,2 +1,22 @@
 -dontobfuscate
 -keepattributes SourceFile, LineNumberTable
+
+# From https://github.com/square/retrofit/blob/master/retrofit/src/main/resources/META-INF/proguard/retrofit2.pro
+
+# 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>
+
+# Keep inherited services.
+-if interface * { @retrofit2.http.* <methods>; }
+-keep,allowobfuscation interface * extends <1>
+
+# 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
+
+# R8 full mode strips generic signatures from return types if not kept.
+-if interface * { @retrofit2.http.* public *** *(...); }
+-keep,allowoptimization,allowshrinking,allowobfuscation class <3>