Issue search support elasticsearch (#9428)

* Issue search support elasticsearch

* Fix lint

* Add indexer name on app.ini

* add a warnning on SearchIssuesByKeyword

* improve code

vendor/github.com/olivere/elastic/v7/plugins.go

@@ -0,0 +1,40 @@
+// Copyright 2012-present Oliver Eilhard. All rights reserved.
+// Use of this source code is governed by a MIT-license.
+// See http://olivere.mit-license.org/license.txt for details.
+
+package elastic
+
+import "context"
+
+// HasPlugin indicates whether the cluster has the named plugin.
+func (c *Client) HasPlugin(name string) (bool, error) {
+	plugins, err := c.Plugins()
+	if err != nil {
+		return false, nil
+	}
+	for _, plugin := range plugins {
+		if plugin == name {
+			return true, nil
+		}
+	}
+	return false, nil
+}
+
+// Plugins returns the list of all registered plugins.
+func (c *Client) Plugins() ([]string, error) {
+	stats, err := c.ClusterStats().Do(context.Background())
+	if err != nil {
+		return nil, err
+	}
+	if stats == nil {
+		return nil, err
+	}
+	if stats.Nodes == nil {
+		return nil, err
+	}
+	var plugins []string
+	for _, plugin := range stats.Nodes.Plugins {
+		plugins = append(plugins, plugin.Name)
+	}
+	return plugins, nil
+}