Adding banner opt

[mikhno-s]

Adding the option to add banner that will be shown to user after connecting to VPN

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Update (if none of the other choices apply)

Checklist

• I have read the CONTRIBUTING.md doc.
• I have added necessary documentation (if appropriate).
• Any dependent changes have been merged and published in downstream modules.