Optional config and prefixed producer config key

[codeliner]

This PR provides some improvements for the factory:

• application config is optional as the producer has smart defaults
• moved config location into a protected method and removed the final keyword from the factory so that a userland implementation can extend the factory and use for example interop-config to load config from a custom path
• changed the config package name from producer to zeromq_producer as we have various producers available for prooph/service-bus which might be used in parallel

[codeliner]

ping @bweston92 please review and merge if you are fine with the changes

[bweston92]

Ok it changes the name of the configuration value which is a BC break but I think it is only me who has used it anyway.

[codeliner]

yeah, we are not stable yet so BC breaks are ok. But I'll add a note in the release.

[sandrokeil]

@codeliner @bweston92 It looks like the interop-config HasDefaultOptions Interface is a perfect match here. I've planned a new release next week and can bring a pull request if you're interest in this.

[bweston92]

@sandrokeil that would be great, that means we wouldn't need the container at all!