MM-10254 Add plugin APIs for getting/updating user statuses (#9101)

* Add plugin APIs for getting/updating user statuses

* Add and update tests

* Updates per feedback

app/plugin_api_test.go

@@ -0,0 +1,32 @@
+// Copyright (c) 2017-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package app
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/mattermost/mattermost-server/model"
+)
+
+func TestPluginAPIUpdateUserStatus(t *testing.T) {
+	th := Setup().InitBasic()
+	defer th.TearDown()
+	api := th.SetupPluginAPI()
+
+	statuses := []string{model.STATUS_ONLINE, model.STATUS_AWAY, model.STATUS_DND, model.STATUS_OFFLINE}
+
+	for _, s := range statuses {
+		status, err := api.UpdateUserStatus(th.BasicUser.Id, s)
+		require.Nil(t, err)
+		require.NotNil(t, status)
+		assert.Equal(t, s, status.Status)
+	}
+
+	status, err := api.UpdateUserStatus(th.BasicUser.Id, "notrealstatus")
+	assert.NotNil(t, err)
+	assert.Nil(t, status)
+}