Include classes that extend from JNA Structure in proguard consumer rules (#6979)

Specifically, I found that `com.sun.jna.Structure` ones via `RustBuffer` that seem to be removed from Android when using [R8 fullMode][0]

Fixes mozilla-mobile/reference-browser#3885 and includes upstreaming the Nimbus RustBuffer rules as well.

[0]: https://developer.android.com/topic/performance/app-optimization/enable-app-optimization#full-mode

Author: jonalmeida

proguard-rules-consumer-jna.pro

@@ -4,4 +4,12 @@
 # See https://github.com/java-native-access/jna/blob/master/www/FrequentlyAskedQuestions.md#jna-on-android
 -dontwarn java.awt.*
 -keep class com.sun.jna.* { *; }
--keepclassmembers class * extends com.sun.jna.* { public *; }
+-keep class * extends com.sun.jna.* { *; }
+-keepclassmembers class * extends com.sun.jna.* { public *; }
+
+####################################################################################################
+# Add explicit keep rules for Nimbus RustBuffer and related structs to avoid
+# overly-aggressive optimization when R8 fullMode is enabled, leading to crashes.
+####################################################################################################
+-keepattributes RuntimeVisibleAnnotations,RuntimeInvisibleAnnotations,RuntimeVisibleTypeAnnotations,RuntimeInvisibleTypeAnnotations,AnnotationDefault,InnerClasses,EnclosingMethod,Signature
+-keep class org.mozilla.experiments.nimbus.internal.** { *; }