keystore: Add support for importing entries from a directory

Required for https://github.com/elastic/cloud-on-k8s/issues/9714

`apm-server keystore add` command is currently done per key. Users may write their own script to import multiple keys. But in wolfi docker image, since it is from chainguard static container, there is no shell, and it is then very difficult to import multiple keys, e.g. in an init container.

Add a way to support importing filename as key and file content as value to keystore.

Start at the apm-server keystore add command and its existing tests to understand how individual keys are imported. Add directory input where each filename becomes a key and its file content becomes the value, then extend the command tests to verify the import behavior.

Written by the indexing model from the issue text.