In order for scripting to be much simpler with `keepassxc-cli show`,
provide a simple --attributesk API which effectively is just a CLI
interface for entry->attributes()->value(...). This allows for more
extensibility and prevents changes in our output formatting from
breaking existing users of keepassxc-cli (if they use --attributes).
Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>