Support new SSL secrets fields

[criamico]

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

Related Kibana issue New secrets added to agent policies with the following tickets:

• [Fleet] Show SSL options for fleet server host in UI kibana#207322
• [Fleet] Show SSL options for ES and remote ES outputs in UI kibana#207326
• [Fleet] Show SSL options for Agent Binary source in UI kibana#207324
• [Fleet] Make proxy certificate key a secret kibana#208748

The new fields to support are:

• For fleet server hosts:secrets.ssl.key (under fleet server inputs section of agent policy)
• For agent binary source: agent.download.secrets.ssl.key
• For outputs: fleet.secrets.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.

[criamico]

Examples of generated policies containing the new secret fields:

ES and remote ES Outputs

fleet.secrets.ssl.key

Agent binary source

Under agent.download.secrets.ssl.key