[feature]Use different cache files for multiple DDNSes

[purerosefallen]

Is your feature request related to a problem? Please describe. When using multiple DDNS instances, the shared cache files would cause the 2nd instance and so on stop working.

Describe the solution you'd like

{
  "cache_file": "/tmp/my_cache"
}

Describe alternatives you've considered Changing the cache filename could also be a solution.

Additional context

[NewFuture]

PR welcomed!