move get_struct_metadata from apache::thrift::detail to apache::thrift namespace
move get_struct_metadata from apache::thrift::detail to apache::thrift namespace

Summary: We made this function public in previous diff.

Reviewed By: iahs

Differential Revision: D26721280

fbshipit-source-id: 6fff81244eed046548df653167288f2b9196508c

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift