Add function to get the system of the service

[rocktavious]

Resolves OpsLevel/terraform-provider-opslevel#567

Problem

There is no way to get the system of a service

Solution

Part 1 for OpsLevel/terraform-provider-opslevel#567

Add a context method on the service resource, similar to that of GetProperties and GetTags

Checklist

• I have run this code, and it appears to resolve the stated issue.
• This PR does not reduce total test coverage
• This PR has no user interface changes or has already received approval from product management to change the interface.
• Does this change require a Terraform schema change?
  • If so what is the ticket or PR #
• Make a changie entry that explains the customer facing outcome of this change

[p25marti]

LGTM! Just the one linting error.