fix: error when updating a linked servers

[edelwud]

Hello,

I prepared this PR to solve problems when updating linked servers. Updating linked servers requires that all servers that have a connection with the one being updated be stopped.

Implemented the following features to solve this problem:

• added the ability to check, add and delete linked servers
• added filtering of routes based on the presence of server links

[disc]

Hello @edelwud, first of all thank you for your contribution. You need to add terraform tests for new functionality to make sure that it works.

[disc]

Hey @edelwud, I appreciate your efforts. Do you need any help with tests?