[NEW RULE ISSUE] Rules should set `classtype`

**What is the issue that you would like `suricata-check` to detect and report on?**
Rules should set `classtype`. An issue should be raised whenever classtype is not set. Exceptions may be made for when `noalert` is set.

**What is the impact of the issue?**
`classtype` is used to infer the priority of alerts raised by rules.

**Describe the solution you'd like**
Add issue `C103`