[Feature Request] Allow Opening Ports on Elastic Agent Container

As it is now, the Elasticsearch container opens port 127.0.0.1:9200, the Kibana container opens port 127.0.0.1:5601, and the Fleet Agent container opens port 127.0.0.1:8220. Unfortunately, the Elastic Agent container does not open any ports at all, and there does not appear to be a way to open one.

It would be nice to have a feature that allows opening custom ports, preferably allowing an externally accessible port like 0.0.0.0:9010, so package development can be done with live data. This would exactly match a production environment, from source, to final indexed documents, instead of relying on a fixed set of sample logs using [Stream](https://github.com/elastic/stream/). This would give the best chance of catching anomalous events during development, as well as a complete A-to-B test of the integration.

The issue names no files, tests, or entry points. Start by tracing how the Elastic Agent container is configured and how existing Elasticsearch, Kibana, and Fleet Agent ports are exposed. Done means a supported way to configure an externally accessible custom port for package development and validation of the resulting workflow.

Written by the indexing model from the issue text.