feat: add custom User-Agent support for subscription and template requests

Add --user-agent / -u CLI flag and user-agent config field to allow
customizing the User-Agent header when fetching subscriptions and
remote templates. This is useful for providers that require a specific
UA to access their subscription links.

Closes #23

util/fetch.go

@@ -5,23 +5,31 @@ import (
 	"net/http"
 )
 
-func Fetch(url string, maxRetryTimes int) (string, error) {
+func Fetch(url string, maxRetryTimes int, userAgent string) (string, error) {
 	retryTime := 0
 	var err error
 	var resp *http.Response
 	for retryTime < maxRetryTimes {
-		resp, err = http.Get(url)
+		req, reqErr := http.NewRequest("GET", url, nil)
+		if reqErr != nil {
+			return "", reqErr
+		}
+		if userAgent != "" {
+			req.Header.Set("User-Agent", userAgent)
+		}
+		resp, err = http.DefaultClient.Do(req)
 		if err != nil {
 			retryTime++
 			continue
 		}
 		var data []byte
 		data, err = io.ReadAll(resp.Body)
+		resp.Body.Close()
 		if err != nil {
 			retryTime++
 			continue
 		}
-		return string(data), err
+		return string(data), nil
 	}
 	return "", err
 }