Implement a reusable GHA workflow to fetch feature flags

[emyller]

In github.com/Flagsmith/flagsmith-ci.

A GitHub Actions reusable workflow is a better solution than an action, as this does need access to the workspace or code.

Input:

• flagsmith_api_url (defaults to https://api.flagsmith.com/)
• flagsmith_admin_api_key
• stale_only (defaults to false)

Output:

• features (response from /api/v1/projects/:project_id/features/)
  If stale_only, filter results by containing any tag of type STALE.

The workflow must error if the request to Flagsmith fails.