scripts: add download-check-artifact.sh script

scripts: add download-check-artifact.sh script

While checking wiki documentation about GPG signatures checking I found
this nice script and I've thought, that it would be nice to provide it
officially instead of some random internet site.

Usage example:

  $ ./scripts/download-check-artifact.sh https://downloads.openwrt.org/snapshots/targets/mediatek/filogic/openwrt-mediatek-filogic-openwrt_one-squashfs-sysupgrade.itb

  1) Downloading image file
  =========================
  ########################################### 100.0%

  2) Downloading checksum file
  ============================
  ########################################### 100.0%

  3) Downloading the GPG signature
  ================================
  ########################################### 100.0%

  4) Verifying GPG signature
  ==========================
  gpg: Signature made Tue 05 Nov 2024 05:21:50 PM UTC
  gpg:                using EDDSA key 92C561DE55AE6552F3C736B82B0151090606D1D9
  gpg: Good signature from "OpenWrt Build System (Nitrokey3) <contact@openwrt.org>" [ultimate]
  Primary key fingerprint: 8A8B C12F 46B8 36C0 F9CD  B36F 1D53 D187 7742 E911
       Subkey fingerprint: 92C5 61DE 55AE 6552 F3C7  36B8 2B01 5109 0606 D1D9

  5) Verifying SHA256 checksum
  ============================
  openwrt-mediatek-filogic-openwrt_one-squashfs-sysupgrade.itb: OK

  Verification done!
  ==================
  Downloaded artifact placed in '/var/home/ynezz/dev/openwrt/openwrt.git/openwrt-mediatek-filogic-openwrt_one-squashfs-sysupgrade.itb'

  Cleaning up.

Adding file in a state as downloaded from https://www.abitare.org/bin/download.sh

References: https://openwrt.org/docs/guide-quick-start/verify_firmware_checksum?s[]=gpg#linux
Signed-off-by: David S. H. Rosenthal <dshr@abitare.org>
Link: https://github.com/openwrt/openwrt/pull/16871
(cherry picked from commit 4c9031fda271ed7d4503642f63ff824faa6cc3a0)
Link: https://github.com/openwrt/openwrt/pull/17097
Signed-off-by: Petr Štetiar <ynezz@true.cz>