Add new API endpoint

[mpociot]

No description provided.

[what-the-diff]

• Added a new controller to return the connected subdomains
• Updated the routes file with this new route
• Created tests for it, including one that checks if we get an empty array when there are no tunnels yet (which is what happens in our current implementation) and another test where we add some fake data to Client::$subdomains so that they show up on /api/tunnels endpoint as well.