feat: add imagespec feature for RegistryMapping

[ANJANA-A-R-K]

Description

Add new image-spec subcommand that wraps the contract-go imagespec
package to generate a Kubernetes pod spec snippet from an OCI image
reference.

Usage:
contract-cli image-spec --in [--container-name ]
[--out ] [--username ] [--password

]

Related Issue

Fixes #

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Code refactor (no functional changes)
• CI/CD or build changes

Target Platform

• HPVS (IBM Hyper Protect Virtual Servers for VPC)
• CCRT (IBM Confidential Computing Container Runtime)
• CCRV (IBM Confidential Computing Container Runtime for Red Hat Virtualization Solutions)
• CCCO (IBM Confidential Computing Containers for Red Hat OpenShift Container Platform)
• All platforms
• Not platform-specific

Testing

• make test passes
• make fmt applied (no formatting changes needed)
• New tests added for new functionality (if applicable)

Checklist

• My code follows the project's coding standards
• I have performed a self-review of my code
• I have added/updated GoDoc comments for public functions
• I have updated documentation (README, docs/README.md) if needed
• All new and existing tests pass (make test)
• I have verified there are no breaking changes (or documented them above)