[executorch][flat_tensor] Generate flatc files (#7521)

Pull Request resolved: https://github.com/pytorch/executorch/pull/7255

1. Move schema files under flat_tensor/serialize
2. Add targets to generate schema files


ghstack-source-id: 260014922
@exported-using-ghexport

Differential Revision: [D66903492](https://our.internmc.facebook.com/intern/diff/D66903492/)