fix: change schema storage definitions

* _picodata_index keys now include space_id
* _picodata_space distribution type changed to "array"
* _picodata_space format type changed to "array"
* add Spaces::insert and Indexes::insert methods
* Ddl now has #[serde(tag = "kind", rename_all = "snake_case")]
* Dml now has #[serde(tag = "op_kind", rename_all = "snake_case")]
* Distribution now has #[serde(tag = "kind", rename_all = "snake_case")]