Setup multi-tenant Server Actor mode

[nicklamonov]

With the migration to PPE pricing we can move the actor to multi-tenant Server Actor mode.

To be evaluated during implementation:

• How to start crawlers based on different inputs and how many we need

Phase 1:

• Resolve how concurrency / variability would work

Phase 2:

• Actual implementation of concurrency / input gates

Notes for the future:

• We need to watch resource consumption on Apify account and possibly later switch to a different user, under which RAG Server will run

[nicklamonov]

Blocked by #60