* Add support for a firmware download proxy. This should help in situations where the player's firmware can't handle https correctly.
Two possibilities:
* full path to image: http://yourlms:9000/plugins/SqueezeESP32/firmware/ESP32-A1S.32.634.master-cmake/squeezelite-esp32-master-cmake-ESP32-A1S-32-V0.634.bin
* use Github's asset ID: http://yourlms:9000/plugins/SqueezeESP32/firmware/34298863
The former is more prone to issues related to the path. A change in the schema could break the matching regex.
The latter is simpler to use if you know the ID. But the ID is not easily available to the user. And it requires one more lookup in the plugin to get from the ID to the download path.
* Add support for proxying firmware downloads through LMS
* add magic asset ID -99 to allow the front-end to check whether the plugin does support download proxying
* web manager is expecting `lms_port` and `lms_ip` in `status.json`. If that's available, check whether plugin does support firmware downloading. If that's the case, download firmwares through LMS
* plugin would cache firmware images. In case of multiple images the file would be served directly from LMS.
Co-authored-by: Michael Herger <michael@herger.net>
