Robtandy/extension point

[robtandy]

This PR attempts to make a clean integration point for this project and customizations.

Such customizations might be:

• adding a new file format
• adding any new PhysicalPlan operator

Such customizations might require the ability to transmit the new physical plan over the wire, so we need to accept a Codec as well for this.

Now DDProxyService and DDWorkerService accept a Customizer trait with three methods to implement to handle this.