Add functionality to have elastic-agent receive data from logstash.

[HGS9761]

Make it possible for elastic-agent to receive data data passes through logstash

The network is segmented (or tiered) hence we need to pass data from one tier to the next.
Also we need to send data to 2 destinations. One of them is Elasticsearch.
It is also necessary to enrich the data before it reaches both destinations.
In order to be able to use the standard integrations using the elastic-agent is necessary.

Elastic-Agent able to receive data sent by logstash.

It is possible to use the tcp input to do this.
But setting it up is a pain.

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[pierrehilbert]

cc @nimarezainia / @flexitrev

[nimarezainia]

Why couldn't another Logstash be used in this case?

[HGS9761]

Well first of all elastic-agent is the future.
More importantly I want to use the standard integrations that come with fleet agents.
The agent can receive all kinds of types of input directly.
I seen no good reason why elastic-agent should not be capable of receiving the same type of input indirectly (be it logstash or another elastic-agent).