Epic: safekeepers dynamic membership change (Phase 1)

[arssher]

We need to be able to move tenants between safekeepers:

1. To migrate all tenants on one safekeeper if it dies.
2. To migrate one tenant if we find that load balancing needs it.

Eventually a proper dynamic membership change two step algorithm should be implemented (), which works correctly regardless of old safekeeper(s) state. However this is complicated, so let's do it in following large steps:

### Tasks
- [ ] #4783
- [ ] https://github.com/neondatabase/neon/issues/6340
- [x] Migration script https://github.com/neondatabase/cloud/pull/15227
- [ ] https://github.com/neondatabase/cloud/issues/7931 required for migration without endpoint restart
- [ ] https://github.com/neondatabase/neon/issues/8700