Check GitHub account storage usage

Summary:
Shared storage for packages or git-lfs is 10 GB per month.

This check queries the GitHub API, using the REST API endpoints for billing,
and raises a warning if we're using more than 8 Go (80%).

This requires authentication. GitHub Token is stored in Vault,
this check uses a new approle devised for monitoring checks.

Reference: https://docs.github.com/en/rest/billing/billing

Test Plan:

• credentials provisioned on WindRiver
• run the check script, get 0 exit code

Reviewers: DorianWinty

Reviewed By: DorianWinty

Differential Revision: https://devcentral.nasqueron.org/D3432