Implement dmverity functionality

[aadhar-agarwal]

Add dm-verity Support to containerd

This PR introduces initial dm-verity support for image layer integrity verification in containerd. Key changes include:

• New internal/dmverity package:

  • Provides Linux-only helpers to format, open, and close dm-verity devices via veritysetup.
  • Includes validation, metadata parsing, and utility functions for managing dm-verity parameters.

• Linux implementation (dmverity_linux.go):

  • Wraps veritysetup CLI commands with configurable options.
  • Adds device and module checks (dm_verity kernel module, veritysetup availability).
  • Handles root hash extraction and device management.

• Non-Linux stub (dmverity_other.go):

  • Returns unsupported errors for non-Linux platforms.

• CI workflow update:

  • Loads dm-verity kernel module and verifies veritysetup version for CI tests.