Added new vCenter Login Banner run command with test coverage

[Kavyareddyguntaka11]

This PR closes WI-36631481

The changes in this PR are as follows:

• Added new vCenter Login Banner run commands:

  • Set-VCLoginBanner
  • Get-VCLoginBanner
  • Remove-VCLoginBanner

• Implemented compatibility and fallback handling for VCSA variants, including:

  • Inline and file-based content configuration
  • Read fallback behavior
  • Toggle verification behavior

• Added and updated Pester test coverage for the new login banner run commands, including:

  • Fallback path validation
  • Failure scenario validation
  • Cleanup guardrails

I have read the contributor guidelines and have completed the following:

• Formatted the code using VS Code default formatter for PowerShell.
• Tested the code end-to-end against an SDDC.
• Documented the functions using standard PowerShell markup and applied AVSAttribute to newly exported functions.