pageserver: fail if id is present in pageserver.toml (#8489)

Overall plan:
https://www.notion.so/neondatabase/Rollout-Plan-simplified-pageserver-initialization-f935ae02b225444e8a41130b7d34e4ea?pvs=4

---

`identity.toml` is the authoritative place for `id` as of
https://github.com/neondatabase/neon/pull/7766

refs https://github.com/neondatabase/neon/issues/7736

control_plane/src/local_env.rs

@@ -514,7 +514,6 @@ impl LocalEnv {
                 #[derive(serde::Serialize, serde::Deserialize)]
                 // (allow unknown fields, unlike PageServerConf)
                 struct PageserverConfigTomlSubset {
-                    id: NodeId,
                     listen_pg_addr: String,
                     listen_http_addr: String,
                     pg_auth_type: AuthType,
@@ -526,18 +525,30 @@ impl LocalEnv {
                         .with_context(|| format!("read {:?}", config_toml_path))?,
                 )
                 .context("parse pageserver.toml")?;
+                let identity_toml_path = dentry.path().join("identity.toml");
+                #[derive(serde::Serialize, serde::Deserialize)]
+                struct IdentityTomlSubset {
+                    id: NodeId,
+                }
+                let identity_toml: IdentityTomlSubset = toml_edit::de::from_str(
+                    &std::fs::read_to_string(&identity_toml_path)
+                        .with_context(|| format!("read {:?}", identity_toml_path))?,
+                )
+                .context("parse identity.toml")?;
                 let PageserverConfigTomlSubset {
-                    id: config_toml_id,
                     listen_pg_addr,
                     listen_http_addr,
                     pg_auth_type,
                     http_auth_type,
                 } = config_toml;
+                let IdentityTomlSubset {
+                    id: identity_toml_id,
+                } = identity_toml;
                 let conf = PageServerConf {
                     id: {
                         anyhow::ensure!(
-                            config_toml_id == id,
-                            "id mismatch: config_toml.id={config_toml_id} id={id}",
+                            identity_toml_id == id,
+                            "id mismatch: identity.toml:id={identity_toml_id} pageserver_(.*) id={id}",
                         );
                         id
                     },

control_plane/src/pageserver.rs

@@ -127,10 +127,13 @@ impl PageServerNode {
         }
 
         // Apply the user-provided overrides
-        overrides.push(
-            toml_edit::ser::to_string_pretty(&conf)
-                .expect("we deserialized this from toml earlier"),
-        );
+        overrides.push({
+            let mut doc =
+                toml_edit::ser::to_document(&conf).expect("we deserialized this from toml earlier");
+            // `id` is written out to `identity.toml` instead of `pageserver.toml`
+            doc.remove("id").expect("it's part of the struct");
+            doc.to_string()
+        });
 
         // Turn `overrides` into a toml document.
         // TODO: above code is legacy code, it should be refactored to use toml_edit directly.