enrich diskqueue configuration with encryption_password when disk queue is encryption is enabled

[leehinman]

Describe the enhancement:

The encryption password for the disk queue is stored locally per instance by elastic-agent. When elastic-agent receives a policy that has an output with disk queue encryption enabled, it needs to generate the password if it doesn't exist or retrieve it if it does from the keystore. Then fill in the queue.disk.encryption_password field with the value before sending to the shipper. If the policy is sent to disk (not the shipper) then the encryption_password should be redacted.

Describe a specific use case for the enhancement or feature:

supporting encrypted disk queues in the shipper.