Ballista cluster sharing by multiple users (with fairness of resource allocation)

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
Let's say we've an organization that deploys ballista cluster to which multiple users connect to. It'd be great if executor slots can be allocated to the users fairly. If a query has consumed all slots allotted to a user, it can't progress until more slots are generated (say by passage of time).

**Describe the solution you'd like**
Each query execution is associated with some identity, identities are rate limited for the slots and tasks for the identities are queued and consumed at allowed rate (or concurrency)

**Describe alternatives you've considered**

**Additional context**
Add any other context or screenshots about the feature request here.

The issue names no files, tests, or entry points. Start by locating Ballista's query identity, executor-slot, and task-scheduling components, then define how per-identity rate or concurrency limits and queuing should work; done means multiple users receive fair allocation while excess tasks wait.

Written by the indexing model from the issue text.