Add segments for bpte

Currently, bpte schema stores program data as bytes inside the flatbuffer. This causes high latency and memory usage when converting llms ~1GB into bpte files, as we go through the flatbuffer json conversion.

We can follow the executorch schema and move the program into segments to avoid the flatbuffer conversion of large byte strings.

Program-data separation is a solution (without weights, program is small) but this is not a guaranteed solution for all users.

Conversion:
https://github.com/pytorch/executorch/blob/ca4e363cb04dec5aa39128220552739c3ba7a4df/devtools/bundled_program/serialize/__init__.py#L82-L100

Schema:
https://github.com/pytorch/executorch/blob/ca4e363cb04dec5aa39128220552739c3ba7a4df/devtools/bundled_program/schema/bundled_program_schema.fbs#L78-L95