Serialize constant data outside of flatbuffer (#2016)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/2016

## Context

This changeset is a mirror of https://github.com/pytorch/executorch/pull/1542 and https://github.com/pytorch/executorch/pull/1543 for the Vulkan delegate, which enables serialization of  constant weight data outside the flatbuffer blob.

I decided to package some miscellaneous codebase improvements with this changeset:

* Update field names of `VkGraph` table in `schema.fbs` and update the namespace to be simply `vkgraph`:
* In `VulkanBackend.cpp`, use aliases for generated flatbuffer types
* Move build targets to a `targets.bzl` file with a `define_common_targets()` method
ghstack-source-id: 215803961
exported-using-ghexport

Reviewed By: mcr229

Differential Revision: D53957852

fbshipit-source-id: 6d6d9c70df6f8cbd9e85e8294208839be84527eb