Please add a PrepareForFirmwareUpdate() method

In fwupd we need to tell the modem to enter firmware update, typically fastboot, but also other protocols will be supported. This means we need to inject something like at+qfastboot into the tty, and MM is in the best place to do that, as it already has the per-vendor quirks and the device open. As well as the D-Bus method, we'd also need something like mm_modem_prepare_for_firmware_update() in the exported API too. It probably needs some kind of PolicyKit protection as otherwise it's a DoS. I'm not fussed by the name, so better ideas welcome. Thanks.