Allow changing the port(s) used by API Security

Problem

When running API Security, it is a common configuration to add a service definition to the job with the target app, database, etc. Sometimes the port numbers conflict with those we use by default, such as port 5000.

This issue has impacted two customers at the time of writing. https://gitlab.zendesk.com/agent/tickets/304578

Proposal

Default to special port 0, but allow override with new variable
Special Port: "0" (means find an available port and use it)
- https://andrewlock.net/how-to-automatically-choose-a-free-port-in-asp-net-core/
Engine writes out URL to file on disk
- worker entry Read the _API variable from a file.

Example Place to Put Write File

public class Startup
{
    public void Configure(IApplicationBuilder app, ILogger<Startup> log)
    {
        // IApplicationBuilder exposes an IFeatureCollection property, ServerFeatures
        var addressFeature = app.ServerFeatures.Get<IServerAddressesFeature>();
        foreach(var address in addressFeature.Addresses)
        {
            _log.LogInformation("Listing on address: " + address);
        }
    }

    // ... other configuration
}

~~Allow the user to change the port using a new variable.~~ ~~1. [ ] Pick an unused port for default (5001)(https://docs.gitlab.com/ee/administration/package_information/defaults.html#ports)~~ ~~2. [ ] Add a new variable to allow the port to be configured by the user at runtime.~~ ~~3. [ ] ...~~

Implementation Proposal

Edited Aug 26, 2022 by Herber Madrigal