Design the Custom Executor Provider

This issue is created to track efforts on designing the interface for Custom Executor Provider.

While #28843 (closed) focuses on choosing a solution for how to make a pluggable integration system for the Custom Executor Provider, I'd like to start a separate discussion about what the exposed interface itself should look like.

References:

• Autoscaling Provider for GitLab Runner to repla... (&2502) (main motivation reason)
• https://docs.gitlab.com/ee/architecture/blueprints/runner_scaling/index.html (Architecture blueprint for the new scaling solution)
• Document internal Executor Interface (!3291 - merged) (documentation of current autoscaling implementation)