fix(ooniprobe): use ooniprobe-cli-unattended for unattended runs (#714)

This diff changes the software name used by unattended runs for which
we did not override the default software name (`ooniprobe-cli`).

It will become `ooniprobe-cli-unattended`. This software name is in line
with the one we use for Android, iOS, and desktop unattended runs.

While working in this diff, I introduced string constants for the run
types and a string constant for the default software name.

See https://github.com/ooni/probe/issues/2081.

internal/ooapi/apimodel/checkin.go

@@ -1,5 +1,7 @@
 package apimodel
 
+import "github.com/ooni/probe-cli/v3/internal/model"
+
 // CheckInRequestWebConnectivity contains WebConnectivity
 // specific parameters to include into CheckInRequest
 type CheckInRequestWebConnectivity struct {
@@ -13,7 +15,7 @@ type CheckInRequest struct {
 	Platform        string                        `json:"platform"`
 	ProbeASN        string                        `json:"probe_asn"`
 	ProbeCC         string                        `json:"probe_cc"`
-	RunType         string                        `json:"run_type"`
+	RunType         model.RunType                 `json:"run_type"`
 	SoftwareName    string                        `json:"software_name"`
 	SoftwareVersion string                        `json:"software_version"`
 	WebConnectivity CheckInRequestWebConnectivity `json:"web_connectivity"`