[Fleet] Add logstash output type in output API and preconfiguration

[nchaulet]

Description

Related to #104987

The output API should support a new output type: logstash

Add a new output type logstash, this output type has the following required properties:

• hosts logstash hosts
• ssl.certificate_authorities server CA
• ssl.certificate for mutual TLS
• ssl.key for mutual TLS

Adding this output to an agent policy will result in the following output in the agent policy YAML

outputs:
  default:
    type:  logstash
    hosts: 127.0.0.1:5044
    ssl.certificate_authorities: 
      - embeded ca 1
      - embeded ca 2
    # For mutual auth
    ssl.certificate: embeded certificate..
    ssl.key: embeded key...

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)