fleet-server listeners

Currently the fleet-server may open the following listeners:
- `0.0.0.0:8220` - default external API port
- `localhost:8221` - internal api port
- `localhost:6060` - optional profile port
- `localhost:5066` - optional metrics port

The external API listener is used by agents on other hosts to connect to fleet

The internal API is used by the agent running fleet

The profile listener is defined in https://github.com/elastic/fleet-server/blob/main/internal/pkg/profile/profile.go and provides the `pprof` endpoints

The metics listener is defined in https://github.com/elastic/fleet-server/blob/main/internal/pkg/api/metrics.go, and uses https://github.com/elastic/elastic-agent-libs/blob/main/api. It appears to also provide the `pprof` endpoints.

We should see if we can consolidate these listeners and better document them.