Part of #3551
Closes #3576

This PR:

• Refactor pipelines classes to make them generic and support pipeline-specific attributes with typing info.
• Implements a gateway pipeline.
• Makes gateway API async and moves termination to the gateway pipeline. Old sync API still works unless DSTACK_FF_PIPELINE_PROCESSING_ENABLED is set. The async deletion is done via a new GatewayModel.to_be_deleted field. Gateway status does not change for a) backward compatibility b) avoiding overriding statuses like FAILED.
• Updates API gateway/service code to avoid running services on to_be_deleted gateways.
• Replaces implicit gateway.* related attributes loads with explicit loads.