Limit size of the search range for VNIs when creating VPCs (#4298)

- Fixes #4283.
- Adds a relatively small limit to the `NextItem` query used for finding
a free VNI during VPC creation. This limits the memory consumption to
something very reasonable, but is big enough that we should be extremely
unlikely to find _no_ available VNIs in the range.
- Add an application-level retry loop when inserting _customer_ VPCs,
which catches the unlikely event that there really are no VNIs
available, and retries a few times.
- Adds tests for the computation of the limited search range.
- Adds tests for the actual exhaustion-detection and retry behavior.

nexus/db-queries/Cargo.toml

@@ -47,6 +47,7 @@ serde_urlencoded.workspace = true
 serde_with.workspace = true
 sled-agent-client.workspace = true
 slog.workspace = true
+static_assertions.workspace = true
 steno.workspace = true
 thiserror.workspace = true
 tokio = { workspace = true, features = [ "full" ] }