Add constant_segment to schema (#1305)

Summary:
Pull Request resolved: https://github.com/pytorch/executorch/pull/1305

Add `constant_segment`, `SubsegmentOffsets` to flatbuffer schema and tests. Used in subsequent diffs.

Generate fixtures with `buck2 run fbcode//executorch/exir/tests:generate_fixtures`

Issue/design: https://github.com/pytorch/executorch/issues/885

Reviewed By: dbort

Differential Revision: D51595437

fbshipit-source-id: 721916bc95af62416b9c32b3032636f2b9de6359