Support fleet server hosts secrets

[criamico]

Part of https://github.com/elastic/ingest-dev/issues/3443

Related Kibana issue: Fleet server hosts secrets will be added with elastic/kibana#207322.

The supported field will be secrets.ssl.key, that maps to ssl.key.

These fields will contain secret references, same as already happens with outputs. Fleet server needs to fetch the secret and insert the values into the mapped fieds before sending the policy to the agent