feat(service): count of issue match by severity for single service

[MR2011]

Task Description
Show the number of critical/high/... issueMatches of a service.

Desired query:

{
  Services{
    edges{
      node{
        id
        issueMatchCounts{
          critical
          high
          medium
          low
        }
      }
    }
  }
}

Acceptance Criteria:

• API returns Service with IssueMatchCounts by severity

Expected Test:

• Test DB query in db test
• E2E test that makes sure API returns expected counts

[MR2011]

We decided to add a issueMatch connection to the service.

See: #514