ocserv: openconnect: T4614: add support for split-dns (equuleus)

[c-po]

Change Summary

set vpn openconnect network-settings split-dns <domain> feature backport

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [x] New feature (non-breaking change which adds functionality)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes)
• [ ] Migration from an old Vyatta component to vyos-1x, please link to related PR inside obsoleted component
• [ ] Other (please describe):

Related Task(s)

• https://phabricator.vyos.net/T4614

Component(s) name

OpenConnect VPN

Proposed changes

How to test

[email protected]:~$ /usr/libexec/vyos/tests/smoke/cli/test_vpn_openconnect.py
test_ocserv (__main__.TestVPNOpenConnect) ... ok

----------------------------------------------------------------------
Ran 1 test in 18.506s

OK

Checklist:

• [x] I have read the CONTRIBUTING document
• [x] I have linked this PR to one or more Phabricator Task(s)
• [x] I have run the components SMOKETESTS if applicable
• [x] My commit headlines contain a valid Task id
• [ ] My change requires a change to the documentation
• [ ] I have updated the documentation accordingly

[sempervictus]

@c-po - i think that adding tunnel-all-dns would fit in scope for this PR since its effectively the inverse of split-dns and only default when a default route is offered over the tunnel.