lib/sign: read ed25519 public keys from well known places
lib/sign: read ed25519 public keys from well known places

If not provided key of file name with keys for remote, then try to use
system defaults:
- /etc/ostree/trusted.ed25519
- /etc/ostree/trusted.ed25519.d/*
- /usr/share/ostree/trusted.ed25519
- /usr/share/ostree/trusted.ed25519.d/*

Signed-off-by: Denis Pynkin <3fbf95ee68899518ee8799d35f88c8f7982aa936@collabora.com>

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree