feat(cpu): add CPUHierarchy_v2 binding with per-CPU serial

[rvatkar]

Adds Go bindings for dcgmGetCpuHierarchy_v2() so callers can access the Grace CPU serial returned by DCGM.

Changes

• Add CPUHierarchy_v2 / CPUHierarchyCPU_v2 support.
• Convert the v2 C hierarchy into Go structs, including Serial.
• Keep the existing v1 CPU hierarchy API unchanged.
• Add converter tests that exercise real C-struct conversion, including:
  • populated serial
  • empty hierarchy / NumCPUs == 0
  • CPU/core conversion coverage

Validation

• go test ./pkg/dcgm