Binary compatibility shims for GeneratedMessageV3 and nested classes

[zhangskz]

Update GeneratedMessageV3 shims for GeneratedMessageV3, Builder, ExtendableMesageOrBuilder, ExtendableMessage, ExtendableBuilder, and FieldAccessorTable classes and gencode-overriden methods that reference those types

Adds compatibility test for generated code jars built against 25.x runtime against newer runtime.