Program.fbs change to support serialized mutable state (#4216)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/4216

Need a way to indicate values that have a meaningful initial state serialized in the program, who also are able to be mutated on device.

https://docs.google.com/document/d/1D8WpMmIiQxU_n5OYWXl3mrpBYUewz79izyAO2UknSsM/edit?usp=sharing

Reviewed By: dbort

Differential Revision: D58747605

fbshipit-source-id: 096b40443ba4ecc8044a4d397838309e8c97c8fa