Add apk Elliptic Curve public key for signing of package indexes

Add apk Elliptic Curve public key for signing of package indexes

apk/openwrt-snapshots.pem contains Elliptic Curve based public key which
is going to be used for signing of apk's package.adb package indexes
after the builds using `apk adbsign --sign-key <key> packages.adb`
command.

apk/openwrt-snapshots.pem.gpg is detached GPG signature produced using
following key from Nitrokey 3A Mini dongle:

 pub   ed25519/0x1D53D1877742E911 2023-05-18 [C] [expires: 2033-05-15]
       Key fingerprint = 8A8B C12F 46B8 36C0 F9CD  B36F 1D53 D187 7742 E911
 uid                   [ultimate] OpenWrt Build System (Nitrokey3) <[email protected]>
 sig 3        0x1D53D1877742E911 2023-05-18  OpenWrt Build System (Nitrokey3) <[email protected]>
 sig          0xCD84BCED626471F1 2023-05-18  OpenWrt Build System (PGP key for unattended snapshot builds) <[email protected]>
 sig          0xCD54E82DADB3684D 2023-05-18  OpenWrt Build System (GnuPGP key for 22.03 release builds) <[email protected]>
 sig          0x88CA59E88F681580 2023-05-18  OpenWrt Build System (PGP key for 21.02 release builds) <[email protected]>
 sub   ed25519/0x2B0151090606D1D9 2023-05-18 [S] [expires: 2033-05-15]
       Key fingerprint = 92C5 61DE 55AE 6552 F3C7  36B8 2B01 5109 0606 D1D9
 sig          0x1D53D1877742E911 2023-05-18  OpenWrt Build System (Nitrokey3) <[email protected]>

References: 6b42a5c8b7dc ("Add new build system signing key stored on Nitrokey 3A Mini dongle")
Signed-off-by: Petr Štetiar <[email protected]>