pc-gadget: document OpenSSL-based secure onboarding hooks (commented reference)

[TSainathPolisetty]

Summary

This PR adds fully commented, non-executing reference implementations
for prepare-device and prepare-serial-request hooks illustrating an
OpenSSL-based secure onboarding flow for Ubuntu Core devices.

All logic is intentionally commented out. There is no functional change
to the existing gadget behavior.

Motivation

This is meant to serve as:

• A readable reference for implementing secure onboarding
• A concrete mapping between the secure onboarding specification and
  hook-level implementation details
• A starting point for future work without changing current defaults

Design notes

• Uses OpenSSL for RSA-2048 key generation and SHA256 signing
• Private key is stored in SNAP_SAVE_DATA and generated once per device
• prepare-device handles identity material only
• prepare-serial-request demonstrates request-id signing and payload construction
• Network operations and snapd defaults are untouched

Scope / Safety

• All code is commented out
• No hooks are executed
• No runtime behavior is modified
• No serial service is contacted

Follow-ups

If useful, this reference can later be:

• Moved to documentation
• Gated behind a build option
• Used as a basis for a TPM-backed variant

[mcw-work]

This PR probably obsoletes PR #111 but does reflect the greater functionality we now have for device registration.