(4/7) Expose connect info for an expired slot's instance

What does this MR do?

• ExpiredConnectInfo(ctx, key): connect info for an expired slot's instance, without activating the slot, so the caller can remove the workload. Returns ErrInstanceStopped when nothing can run on the instance.
• ExpiredSlot.InstanceStopped tells the caller up front which expired slots are on a stopped instance.

Why was this MR needed?

Part 4 of 7. The runner has to reach the instance to remove an expired environment before the slot is freed.

What's the best way to test this MR?

go test -race ./.... TestExpiredConnectInfo.

What are the relevant issue numbers?

Part of the stack started in !117 (merged).

MR stack

Merge in order:

1. (1/7) Give acquired slots an optional expiry (!117 - merged) • Ashvin Sharma
2. (2/7) Let an instance mark a suspended slot exp... (!119 - merged) • Ashvin Sharma
3. (3/7) Expire suspended slots past their expiry (!120 - merged) • Ashvin Sharma
4. (4/7) Expose connect info for an expired slot's... (!121 - merged) • Ashvin Sharma (this MR)
5. (5/7) Free expired slots once their workload is... (!122 - merged) • Ashvin Sharma
6. (6/7) Remove stopped instances whose slots have... (!123 - merged) • Ashvin Sharma
7. (7/7) Refuse to resume a suspended slot past it... (!118 - merged) • Ashvin Sharma