MM-26056 Add plugin ability to execute slash command. (#14898)

* Add plugin ability to execute slash command.

* A test

* Add site url to command args for plugin API

* Remove unessiary complexity.

app/plugin_api_test.go

@@ -1663,6 +1663,27 @@ func TestPluginHTTPUpgradeWebSocket(t *testing.T) {
 	}
 }
 
+func TestPluginExecuteSlashCommand(t *testing.T) {
+	th := Setup(t).InitBasic()
+	defer th.TearDown()
+	api := th.SetupPluginAPI()
+
+	newUser := th.CreateUser()
+	th.LinkUserToTeam(newUser, th.BasicTeam)
+
+	t.Run("run invite command", func(t *testing.T) {
+		_, err := api.ExecuteSlashCommand(&model.CommandArgs{
+			Command:   "/invite @" + newUser.Username,
+			TeamId:    th.BasicTeam.Id,
+			UserId:    th.BasicUser.Id,
+			ChannelId: th.BasicChannel.Id,
+		})
+		require.NoError(t, err)
+		_, err2 := th.App.GetChannelMember(th.BasicChannel.Id, newUser.Id)
+		require.Nil(t, err2)
+	})
+}
+
 func TestPluginAPISearchPostsInTeamByUser(t *testing.T) {
 	th := Setup(t).InitBasic()
 	defer th.TearDown()