feat(resharding): set defaults for state split config (#10202)

Without the individual defaults for fields it was impossible to set only
some fields while leaving others out. Now by setting only some fields
the rest will get the sensible defaults. It was annoying for mocknet and
would be for node maintainers if they need to adjust this config.

core/chain-configs/src/client_config.rs

@@ -163,6 +163,7 @@ impl SyncConfig {
 }
 
 #[derive(serde::Serialize, serde::Deserialize, Clone, Copy, Debug)]
+#[serde(default)]
 pub struct StateSplitConfig {
     /// The soft limit on the size of a single batch. The batch size can be
     /// decreased if resharding is consuming too many resources and interfering