[MM-25659] ability to permanent delete channel through client (#15202)

Summary

    Ability to permanent delete channel through client

Ticket Link

    https://mattermost.atlassian.net/browse/MM-25659

api4/channel.go

@@ -1074,7 +1074,15 @@ func deleteChannel(c *Context, w http.ResponseWriter, r *http.Request) {
 		return
 	}
 
-	err = c.App.DeleteChannel(channel, c.App.Session().UserId)
+	if c.Params.Permanent {
+		if *c.App.Config().ServiceSettings.EnableAPIChannelDeletion {
+			err = c.App.PermanentDeleteChannel(channel)
+		} else {
+			err = model.NewAppError("deleteChannel", "api.user.delete_channel.not_enabled.app_error", nil, "channelId="+c.Params.ChannelId, http.StatusUnauthorized)
+		}
+	} else {
+		err = c.App.DeleteChannel(channel, c.App.Session().UserId)
+	}
 	if err != nil {
 		c.Err = err
 		return