chore: set User-Agent on all outgoing HTTP requests (#10867)

It's only polite, and it enables better troubleshooting in situations
like weird-ass requests to the upgrades download service.

---------

Signed-off-by: Jakob Borg <jakob@kastelo.net>

lib/relay/client/dynamic.go

@@ -14,6 +14,7 @@ import (
 	"sync"
 	"time"
 
+	"github.com/syncthing/syncthing/lib/build"
 	"github.com/syncthing/syncthing/lib/osutil"
 	"github.com/syncthing/syncthing/lib/rand"
 	"github.com/syncthing/syncthing/lib/relay/protocol"
@@ -53,6 +54,7 @@ func (c *dynamicClient) serve(ctx context.Context) error {
 		l.Debugln(c, "failed to lookup dynamic relays", err)
 		return err
 	}
+	req.Header.Set("User-Agent", build.UserAgent())
 	data, err := http.DefaultClient.Do(req)
 	if err != nil {
 		l.Debugln(c, "failed to lookup dynamic relays", err)