MM-53924 - Implement push notifications plugin hook and plugin api method (#24350)

* Revert "MM-52804 - Implement SendPushNotification plugin api method (#24273)"

This reverts commit 8418eefb75.

* Revert "MM-53924 - Implement NotificationWillBePushed plugin hook (#24263)"

This reverts commit f13a531bca.

* implement NotificationWillBePushed plugin hook

* implement SendPushNotification plugin api method

* move where we're setting post and channel type

* fix comment

---------

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

server/channels/app/plugin_hooks_test.go

@@ -1344,20 +1344,28 @@ func TestHookNotificationWillBePushed(t *testing.T) {
 	}
 
 	tests := []struct {
-		name                  string
-		testCode              string
-		expectedNotifications int
+		name                        string
+		testCode                    string
+		expectedNotifications       int
+		expectedNotificationMessage string
 	}{
 		{
 			name:                  "successfully pushed",
-			testCode:              `return false`,
+			testCode:              `return nil, ""`,
 			expectedNotifications: 6,
 		},
 		{
 			name:                  "push notification rejected",
-			testCode:              `return true`,
+			testCode:              `return nil, "rejected"`,
 			expectedNotifications: 0,
 		},
+		{
+			name: "push notification modified",
+			testCode: `notification.Message = "brand new message"
+	return notification, ""`,
+			expectedNotifications:       6,
+			expectedNotificationMessage: "brand new message",
+		},
 	}
 	for _, tt := range tests {
 		t.Run(tt.name, func(t *testing.T) {
@@ -1440,7 +1448,11 @@ func TestHookNotificationWillBePushed(t *testing.T) {
 				case model.PushTypeMessage:
 					numMessages++
 					assert.Equal(t, th.BasicChannel.Id, n.ChannelId)
-					assert.Contains(t, n.Message, "mentioned you")
+					if tt.expectedNotificationMessage != "" {
+						assert.Equal(t, tt.expectedNotificationMessage, n.Message)
+					} else {
+						assert.Contains(t, n.Message, "mentioned you")
+					}
 				default:
 					assert.Fail(t, "should not receive any other push notification types")
 				}