rpc: add separate size limit for websocket (#22385)

This makes the WebSocket message size limit independent of the
limit used for HTTP requests. The new limit for WebSocket messages 
is 15MB.

rpc/testservice_test.go

@@ -20,6 +20,7 @@ import (
 	"context"
 	"encoding/binary"
 	"errors"
+	"strings"
 	"sync"
 	"time"
 )
@@ -194,3 +195,12 @@ func (s *notificationTestService) HangSubscription(ctx context.Context, val int)
 	}()
 	return subscription, nil
 }
+
+// largeRespService generates arbitrary-size JSON responses.
+type largeRespService struct {
+	length int
+}
+
+func (x largeRespService) LargeResp() string {
+	return strings.Repeat("x", x.length)
+}