[CPU] Implement the runtime executor

[GregoryComer]

Implement the runtime executor for the XNNPACK/CPU delegate.

Part of #19863.

[GregoryComer]

Implemented across stacked PRs:

• XNNPACK graph runtime: executor engine #20254 — executor engine
• XNNPACK graph runtime: execution planning #20252 — execution planning
• XNNPACK graph runtime: arena allocator and shape environment #20250 — arena allocator and shape environment
• XNNPACK graph runtime: stub in-tree operator interface #20249 — in-tree operator interface