[API] Verify VPN Credentials

[tzeejay]

Usage: used to verify that the VPN credentials on this server still exist and are still valid (servers could have a hardware failure and we'd recover them with the same hostname, but with the VPN credentials missing. That needs to be mitigated, or a client could not have connected to our infrastructure for weeks at which point the VPN credentials could have been cleaned up by our system due to inactivity)

GuardianConnect ref implementation: https://github.com/GuardianFirewall/GuardianConnect/blob/main/GuardianConnect/Classes/API/GRDGatewayAPI.m#L376

HTTP method: POST
API endpoint: /api/v1.3/device/:deviceid/verify-credentials

Request body:

{
    "subscriber-credential": "....",
    "api-auth-token": "..."
}

Possible error HTTP codes: 500, 400
If an error HTTP code is returned, a JSON object is returned in the body as well eg.

{
    "error-title": "...",
    "error-message": "..."
}

Success HTTP code: 200 Ok