storcon: improve resource rebalancing across Pageservers

[erikgrinaker]

During e.g. releases, tenant shards are moved around as Pageservers restart. This can lead to load imbalances, where some Pageservers have much higher load than others, and can see resource exhaustion. This can also happen gradually as tenant workloads change.

Currently, such imbalances must be resolved manually, by moving tenants around. This is slow, laborious, and often doesn't happen at all.

The storage controller should monitor Pageserver resource usage and attempt to balance resource usage and avoid overload across the following dimensions:

• CPU usage
• Memory usage
• Disk IOPs
• Disk bandwidth
• Disk space
• Tenant counts
• GetPage request rate
• WAL ingestion rate

This must be combined with other constraints such as AZ affinity.