TUN-8725: implement diagnostic procedure

## Summary
Add a function that orchestrates the diagnostic jobs producing a zip file at the end.

Closes TUN-8725

Author: lmpn

diagnostic/client.go

@@ -38,7 +38,7 @@ func (client *httpClient) SetBaseURL(baseURL *url.URL) {
 
 func (client *httpClient) GET(ctx context.Context, endpoint string) (*http.Response, error) {
 	if client.baseURL == nil {
-		return nil, ErrNoBaseUrl
+		return nil, ErrNoBaseURL
 	}
 	url := client.baseURL.JoinPath(endpoint)
 

diagnostic/consts.go

@@ -22,4 +22,12 @@ const (
 	goroutineDumpEndpoint       = "debug/pprof/goroutine"
 	metricsEndpoint             = "metrics"
 	tunnelConfigurationEndpoint = "/config"
+	// Base for filenames of the diagnostic procedure
+	systemInformationBaseName = "systeminformation.json"
+	metricsBaseName           = "metrics.txt"
+	zipName                   = "cloudflared-diag"
+	heapPprofBaseName         = "heap.pprof"
+	goroutinePprofBaseName    = "goroutine.pprof"
+	networkBaseName           = "network.json"
+	tunnelStateBaseName       = "tunnelstate.json"
 )