feat: remove owner from _pico_index table

Summary

Remove owner from _pico_index table, rename owner to initiator in CreateIndexParams and Ddl::CreateIndex

 enum Ddl {
     CreateIndex {
-        owner: UserId,
+        initiator: UserId,
     },
 }

 pub struct IndexDef {
-    pub owner: UserId,
 }


 pub struct CreateIndexParams {
-    pub(crate) owner: UserId,
+    pub(crate) initiator: UserId,
 }

Changelog is already up to date:

System table _pico_index format changed, ..., other fields were rearranged significantly, see [Architecture — System tables]

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• Close #676 (closed)
• Cherry-pick to: none / 24.2 / 24.3
• Docs follow-up: docs#292 (closed)