sstp: T4393: Add support to configure host-name (SNI) (backport #3436)

[mergify]

Change Summary

sstp: Add support to configure host-name (SNI)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• 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://vyos.dev/T4393

Related PR(s)

vyos/vyos-documentation#1432

Component(s) name

vpn sstp

Proposed changes

How to test

set vpn sstp host-name example.vyos.io
commit

vyos@vyos# cat /run/accel-pppd/sstp.conf | grep host-name
host-name=example.vyos.io

Smoketest result

vyos@vyos# /usr/libexec/vyos/tests/smoke/cli/test_vpn_sstp.py
test_accel_ipv4_pool (__main__.TestVPNSSTPServer.test_accel_ipv4_pool) ... ok
test_accel_ipv6_pool (__main__.TestVPNSSTPServer.test_accel_ipv6_pool) ... ok
test_accel_limits (__main__.TestVPNSSTPServer.test_accel_limits) ... ok
test_accel_local_authentication (__main__.TestVPNSSTPServer.test_accel_local_authentication) ... ok
test_accel_name_servers (__main__.TestVPNSSTPServer.test_accel_name_servers) ... ok
test_accel_next_pool (__main__.TestVPNSSTPServer.test_accel_next_pool) ... ok
test_accel_ppp_options (__main__.TestVPNSSTPServer.test_accel_ppp_options) ... ok
test_accel_radius_authentication (__main__.TestVPNSSTPServer.test_accel_radius_authentication) ... ok
test_accel_shaper (__main__.TestVPNSSTPServer.test_accel_shaper) ... ok
test_accel_snmp (__main__.TestVPNSSTPServer.test_accel_snmp) ... ok
test_accel_wins_server (__main__.TestVPNSSTPServer.test_accel_wins_server) ... ok
test_sstp_host_name (__main__.TestVPNSSTPServer.test_sstp_host_name) ... ok

----------------------------------------------------------------------
Ran 12 tests in 67.336s

OK

Checklist:

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

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This is an automatic backport of pull request #3436 done by [Mergify](https://mergify.com).