Avoid per-file HTTP HEAD requests during config validation when running
esphome logs against a previously-cached project. The skip_external_update
flag was already plumbed for git operations, but external_files.download_content
ignored it. Thread it through CORE so audio_file, micro_wake_word,
speaker/media_player, image, font, and bme68x_bsec2 reuse cached files
without per-URL network round-trips when the file already exists locally.