feat: Add support of signature verification for attestation records in decryptAttestation record command

[vikas-sharma24]

Description

Brief description of the changes

Related Issue

Fixes #issue_number

Type of Change

• Bug fix
• New feature
• Breaking change
• Documentation update
• Code refactor

Testing

vikassharma@Vikass-MacBook-Pro contract-cli % ./contract-cli decrypt-attestation --in se-checksums.txt.enc --priv private-key.pem --signature se-signature.bin --attestation-cert attestation.crt
Signature verification successful - attestation records are valid and signed by IBM

26.2.0
Machine Type/Plant/Serial: xxxxx
Image age: xx days since creation.
Encryption Certificate valid until: xxxxx
Attestation Certificate valid until: xxxxx
xxxxx certificate_expiry_date.json
xxxxx root.tar.gz
xxxxx baseimage
xxxxx sbom
xxxxx /dev/disk/by-label/cidata
xxxxx cidata/meta-data
xxxxx cidata/user-data
xxxxx cidata/vendor-data
xxxxx contract:env 
xxxxx contract:attestationPublicKey 
xxxxx contract:workload

vikassharma@Vikass-MacBook-Pro contract-cli % ./contract-cli decrypt-attestation --help 
Usage:
  contract-cli decrypt-attestation [flags]

Mandatory Flags:
  --in                Path to encrypted attestation file (se-checksums.txt.enc, use '-' for standard input)
  --priv              Path to private key file for signing

Optional Flags:
  --attestation-cert  Path to IBM attestation certificate file (PEM format)
  --help              help for decrypt-attestation
  --out               Path to save decrypted attestation records
  --signature         Path to signature file (se-signature.bin)

vikassharma@Vikass-MacBook-Pro contract-cli % ./contract-cli decrypt-attestation --in se-checksums.txt.enc --priv private-key.pem --signature se-signature.bin                                                              
Error: --signature and --attestation-cert flags must be used together. Both are required or both should be omitted.

Usage:
  contract-cli decrypt-attestation [flags]

Flags:
      --attestation-cert string   Path to IBM attestation certificate file (PEM format)
  -h, --help                      help for decrypt-attestation
      --in string                 Path to encrypted attestation file (se-checksums.txt.enc, use '-' for standard input)
      --out string                Path to save decrypted attestation records
      --priv string               Path to private key file for signing
      --signature string          Path to signature file (se-signature.bin)

Checklist

• My code follows the project's coding standards
• I have performed a self-review of my code
• I have commented my code where necessary
• I have updated the documentation
• I have added tests that prove my fix/feature works
• All new and existing tests pass

[Sashwat-K]

• Add example in docs/README.md
• Use log.Fatal() for error statements

[vikas-sharma24]

@Sashwat-K updated decrypt-attestation doc with updated examples.

[Sashwat-K]

Looks good to me. Thank you @vikas-sharma24 for this feature!

[github-actions]

🎉 This PR is included in version 1.20.0 🎉

The release is available on GitHub release

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