Separate domain config source

Description

This is my second attempt to !186 (closed).

This merge request tries to extract a code that we use to fetch a domain configuration to a separate package.

The plan is to have three packages source <- domain <- serving.

• source is responsible for getting a domain config and creating an instance of a domain
• domain contains all the necessary things to serve a domain using a storage / serving driver for this particular domain
• serving driver will be responsible for serving a domain
  • initially only by reading files from the disk using storage/disk
  • we can implement object storage later
  • we will implement serverless serving after the new source/gitlab is implmented