XNNPACK graph runtime: flatbuffer graph deserialization

Adds FlatbufferGraphBuilder, which deserializes the XNNPACK .pte payload into
the in-memory graph IR and reports external input/output ids.

Authored with Claude.

ghstack-source-id: 20e083b
ghstack-comment-id: 4695612750
Pull-Request: #20255

Author: GregoryComer