rainbow: add Server header (reusing User-Agent)

cmd/rainbow/main.go

···
 			CollectionDirHost: collectionDirHost,
 			CursorFile:        cctx.String("cursor-file"),
 			PebbleOptions:     &ppopts,
-			UserAgent:         fmt.Sprintf("rainbow/%s", versioninfo.Short()),
+			UserAgent:         fmt.Sprintf("rainbow/%s (atproto-relay)", versioninfo.Short()),
 		}
 		spl, err = splitter.NewSplitter(conf, nextCrawlers)
 	} else {

splitter/splitter.go

···
 		AllowHeaders: []string{echo.HeaderOrigin, echo.HeaderContentType, echo.HeaderAccept, echo.HeaderAuthorization},
 	}))
 
+	e.Use(func(next echo.HandlerFunc) echo.HandlerFunc {
+		return func(c echo.Context) error {
+			c.Response().Header().Set(echo.HeaderServer, s.conf.UserAgent)
+			return next(c)
+		}
+	})
+
 	/*
 		if !s.ssl {
 			e.Use(middleware.LoggerWithConfig(middleware.LoggerConfig{