storage controller: quality of life improvements for AZ handling (#10379)

## Problem

Since https://github.com/neondatabase/neon/pull/9916, the preferred AZ
of a tenant is much more impactful, and we would like to make it more
visible in tooling.

## Summary of changes

- Include AZ in node describe API
- Include AZ info in node & tenant outputs in CLI
- Add metrics for per-node shard counts, labelled by AZ
- Add a CLI for setting preferred AZ on a tenant
- Extend AZ-setting API+CLI to handle None for clearing preferred AZ

storage_controller/src/node.rs

@@ -299,6 +299,7 @@ impl Node {
             id: self.id,
             availability: self.availability.clone().into(),
             scheduling: self.scheduling,
+            availability_zone_id: self.availability_zone_id.0.clone(),
             listen_http_addr: self.listen_http_addr.clone(),
             listen_http_port: self.listen_http_port,
             listen_pg_addr: self.listen_pg_addr.clone(),