[xnnpack][lite-int][3/n] flatbuffer serializer class (#87907)
Creating a serializer class that allows us to serialize the xnnpack graph creation arguments. This essentially abstracts away the flatbuffer api manipulation and serialization that we deal with.
As a result we can call
```
XNNSerializer::serializeAddNode()
XNNSerializer::serializeTensorValue()
XNNSerializer::finishAndSerialize
```
to serialize the graph
Differential Revision: [D39196312](https://our.internmc.facebook.com/intern/diff/D39196312/)
**NOTE FOR REVIEWERS**: This PR has internal Meta-specific changes or comments, please review them on [Phabricator](https://our.internmc.facebook.com/intern/diff/D39196312/)!
Pull Request resolved: https://github.com/pytorch/pytorch/pull/87907
Approved by: https://github.com/digantdesai
