Extension specification of python/system dependencies

[brollb]

DeepForge extensions (such as https://github.com/deepforge-dev/deepforge-keras) should be able to specify dependencies so the deepforge deployment can be updated accordingly on installation.

This is related to #1402 and #1392

[brollb]

As the current plan is to use miniconda (#1402) and there are 3 different types of dependencies in deepforge (server, worker, and job-specific - the former 2 are the most relevant for this comment), extensions could specify their dependencies in conda environment files such as:

• environment.server.yml for server dependencies
• environment.worker.yml for worker dependencies
• alternatively/optionally, environment.yml could be specified for common dependencies across the worker and server