[WIP] Local Execution using Custom Worker Containers

[teo-milea]

Description

We have implemented local execution but it only works for in-memory nodes with in-memory workers. This limits the DS to test their codes with custom worker images, which we expect to be heavily used when users need some custom libraries that are not included with PySyft.

Affected Dependencies

How has this been tested?

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [x] I have labeled this PR with the relevant Type labels
• [x] My changes are covered by tests