Always on service location for Linux

[kchudy]

• use the existing service
• store config in /etc/defguard
• core UI should inform that Always on is for windows and linux
• Requires changes in the Core UI in the following screens (Always On available for Linux and Windows, Prelogon available on Windows):

- send Always on service locations to linux clients in version 2.1 - service locations should not be visible in the client UI

[j-chmielewski]

@jakub-tldr test scenarios:

• Core: setup always-on service location and connect gateway
• Test linux client:
  • Enroll linux client
  • Client system should have appropriate wg interface automatically configured on system start, even before defguard client UI is launched
  • Client system should be able to ping service location gateway
  • Gateway system should be able to ping the client
• Test windows client:
  • Enroll windows client
  • Client system should have appropriate wg interface automatically configured on system start, even before defguard client UI is launched
  • Client system should be able to ping service location gateway
  • Gateway system should be able to ping the client