Create Configuration Object

[jhpierce]

Description

Create a Configuration Pydantic model, which holds all of the values needed for a Gator "run". These fields include:

• changeset - a Changeset object
• dry_run A boolean indicating whether or not this is a dry run
• github_domain
• github_token_env_var the name of the environment variable where a Github token is stored
• repositories a list of repository slugs to process, of the form some-org/some-repo

This should be defined in a module gator.models