🚀 Add KeyVaultKeyReference parameter for GitHub App authentication (#63)

This PR adds support for authenticating GitHub Apps using Azure KeyVault key references, enabling secure private key storage in Azure KeyVault instead of GitHub secrets.

Changes Made

🔧 Core Implementation

• action.yml: Added new KeyVaultKeyReference input parameter with proper description and environment variable mapping
• scripts/init.ps1:
  • Added validation logic ensuring mutual exclusion between PrivateKey and KeyVaultKeyReference
  • Added requirement validation that ClientID must be provided with exactly one key parameter
  • Implemented new authentication path: Connect-GitHub -ClientID <ClientID> -KeyVaultKeyReference <url>
  • Updated module status reporting to include KeyVault key reference status

📚 Documentation

• README.md:
  • Added KeyVaultKeyReference to inputs documentation table
  • Added comprehensive Example 5 demonstrating Azure KeyVault authentication workflow
  • Updated example numbering consistently
  • Included note about required azure/login action for KeyVault authentication

Usage Example

jobs:
  Run-Script:
    runs-on: ubuntu-latest
    steps:
      - name: Login to Azure
        uses: azure/login@v1
        with:
          creds: ${{ secrets.AZURE_CREDENTIALS }}

      - name: Run script
        uses: PSModule/GitHub-Script@v1
        with:
          ClientID: ${{ secrets.CLIENT_ID }}
          KeyVaultKeyReference: ${{ secrets.KEYVAULT_KEY_REFERENCE }}
          Script: |
            LogGroup "Get-GitHubApp" {
              Get-GitHubApp
            }

Validation

The implementation includes comprehensive validation:

• ✅ Mutual exclusion: Only one of PrivateKey or KeyVaultKeyReference can be provided
• ✅ Requirement validation: ClientID must be provided with exactly one key parameter
• ✅ Error handling for invalid input combinations
• ✅ PowerShell and YAML syntax validation
• ✅ Integration testing with 6/6 test scenarios passing

Authentication Flow

The action now supports three authentication methods:

1. Token: Token parameter (existing)
2. GitHub App with Private Key: ClientID + PrivateKey (existing)
3. GitHub App with KeyVault: ClientID + KeyVaultKeyReference (new)

Fixes #62.

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