Allow downloading `module_firmware_index.json` and signature in a single tar.bz2 archive

[umbynos]

Similar to arduino/arduino-cli#1734 but for fwuploader and module_firmware_index.json
Originally reported by @kittaakos:

updatableBoards failed with error: Error executing "/Users/a.kitta/dev/git/arduino-ide/arduino-ide-extension/build/arduino-fwuploader" firmware list --format json: openpgp: invalid signature: hash tag doesn't match Error: Error executing "/Users/a.kitta/dev/git/arduino-ide/arduino-ide-extension/build/arduino-fwuploader" firmware list --format json: openpgp: invalid signature: hash tag doesn't match

@cmaglie theory:

the hypothesis is that the cloudflare cache is not aligned for some millisecond and if you download the signature in that instant the signature do not match

probably the most reliable solution is to do what we did with the package_index -> create an tar.bz2 archive with both signature and index, so they are downloaded togherter