Added Github "source version" metric

[termeczky]

As a Quality-time user I want to to have access to "source version" so that track this metric within Quality-time

Acceptance criteria

• A Quality-time user can add GitHub as source to a "source version" metric.

Assumptions

• Github exposes an endpoint to retrieve the GitHub version.

Tasks

Components

Shared code

• Extend the example reports
• Indicate that GitHub supports the source version metric in the data model.

Collector

• Add a GitHubSourceVersion collector in source_collectors/github/source_version.py.

Documentation

• Update changelog