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)
• [x] 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

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