feat(vps): filter Operating Systems without needing a VPS.

[JoooostB]

Short description

Adds filter API endpoint to allow for retrieving Operating Systems without needing a VPS. This can be particularly handy when no VPS is created yet, as is currently an open issue for the Terraform provider: aequitas/terraform-provider-transip#74

Checklist

• I read the CONTRIBUTING.md document
• This code actually compiles
• This code solves my problem
• I've updated the inline documentation
• I added or modified test(s) to prevent this issue from ever occuring again

[JoooostB]

I was in doubt if this was completely related to the VPS package, or a separate package had te be made as the URL does not start with /vps/. In my understanding it's sole purpose is to gather information about the VPS product, so I added it to the VPS package for now.

[JustInVTime]

@svenwiltink What needs to be done to get this merged and released?

The transip terraform module is broken now, for creating a new VPS. (aequitas/terraform-provider-transip#88). By adding the operating systems api call to this library, the terraform module can be fixed.

[svenwiltink]

Thanks for the reminder. This has disappeared from my radar, but I'll take a look this week or the next. It should be rather straightforward but am unfortunately busy as well