Add Signer classes to consumer proguard rules (#3310)

This prevents the R8 full-mode optimizer from removing the constructors for these classes since they are instantiated via reflection.

Author: mattcreaser

aws-android-sdk-core/consumer-proguard-rules.pro

@@ -17,3 +17,9 @@
 # The SDK has several references of Apache HTTP client
 -dontwarn com.amazonaws.http.**
 -dontwarn com.amazonaws.metrics.**
+
+# AGP 8 enables R8 full-mode optimization, which will remove constructors of classes that are only
+# instantiated via reflection. These classes are instantiated via reflection in the SignerFactory.
+-keep class com.amazonaws.auth.AWS4Signer { *; }
+-keep class com.amazonaws.auth.QueryStringSigner { *; }
+-keep class com.amazonaws.auth.NoOpSigner { *; }