message: add java2 as a thrift language
  Summary:
  in this diff, we add java2 as a thrift language.
  java2 is newer codegen with better features not possible on java-swift stack. it is the future default for java thrift.
diff: namespace java com.facebook.fb303.core
  namespace java.swift com.facebook.swift.fb303.core
  +namespace java2 com.facebook.thrift.fb303.core
  namespace cpp facebook.fb303
  namespace py.asyncio fb303_asyncio.fb303_core
  namespace perl fb303
  