MM-49984: Displaying active search backend in system console (#22721)

This will be shown in the system console to let users know
which search backend is active.

Implemented via adding an extra param in the ping response.

https://mattermost.atlassian.net/browse/MM-49984

```release-note
The database section in the system console now has an additional
read-only section which shows the active search backend in use.

This can be helpful to confirm which is the currently active
search engine when there are multiple of them configured.
```
---------

Co-authored-by: Mattermost Build <build@mattermost.com>

server/platform/services/searchengine/searchengine_test.go

@@ -0,0 +1,42 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package searchengine
+
+import (
+	"testing"
+
+	"github.com/mattermost/mattermost-server/v6/model"
+	"github.com/mattermost/mattermost-server/v6/server/platform/services/searchengine/mocks"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestActiveEngine(t *testing.T) {
+	cfg := &model.Config{}
+	cfg.SetDefaults()
+
+	b := NewBroker(cfg)
+
+	esMock := &mocks.SearchEngineInterface{}
+	esMock.On("IsActive").Return(true)
+	esMock.On("GetName").Return("elasticsearch")
+
+	bleveMock := &mocks.SearchEngineInterface{}
+	bleveMock.On("IsActive").Return(true)
+	bleveMock.On("IsIndexingEnabled").Return(true)
+	bleveMock.On("GetName").Return("bleve")
+
+	assert.Equal(t, "database", b.ActiveEngine())
+
+	b.ElasticsearchEngine = esMock
+	assert.Equal(t, "elasticsearch", b.ActiveEngine())
+
+	b.ElasticsearchEngine = nil
+	b.BleveEngine = bleveMock
+	assert.Equal(t, "bleve", b.ActiveEngine())
+
+	b.BleveEngine = nil
+	*b.cfg.SqlSettings.DisableDatabaseSearch = true
+
+	assert.Equal(t, "none", b.ActiveEngine())
+}