openwrt-keyring: add apk 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 on the buildbot.

References: openwrt/buildbot#46
Link: openwrt#16539
Signed-off-by: Petr Štetiar <[email protected]>