Add vCenter Login Banner run-command support with fallback and tests

[Kavyareddyguntaka11]

This PR closes

WI- 36631481

Summary

This PR adds vCenter Login Banner support to AVS Run Command.

The changes in this PR are as follows

• Added new login banner functions:
  • Set-VCLoginBanner
  • Get-VCLoginBanner
  • Remove-VCLoginBanner
• Added compatibility fallback for older VCSA command behavior:
  • Content set fallback (inline -> file)
  • Consent fallback (true/false -> Y/N)
  • Enable verification fallback (if enable fails, verify state via read commands)
• Added Pester tests
• Manually validated commands on test vCenter and confirmed UI banner behavior

Validation done

• Verified banner set flow on VCSA variant where:
  • -content is unsupported
  • -enable_checkbox true/false is unsupported
  • -enable true may fail
• Verified fallback flow succeeds when banner state is already enabled
• Verified hard-stop behavior when banner is not enabled or cannot be verified

Checklist

• Formatted the code using VSCode 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.