Store queries in schema template and warmup plan cache on startup

[sergei-pustovykh]

This PR introduces a new concept: RecordLayerSchemaTemplate can declare a set of SELECT queries — "stored queries" - which are populated into the RecordMetaData and serialized to protobuf alongside the rest of the schema template.
To initialise schema template, a new clause CREATE QUERY name AS <select> is introduced, valid only inside CREATE SCHEMA TEMPLATE. The body accepts any SELECT-shaped query. DML and DDL bodies are rejected at parse time. The original SQL text is captured (whitespace and casing preserved) and stored on the template. On the wire it surfaces as a stored_queries map<string, string> field on MetaData in record_metadata.proto.
On engine startup, OfflineStoredQueriesProcessor walks the catalog, iterates every schema template, and plans each of its stored queries offline automatically inserting the resulting plans into the RelationalPlanCache.
Planner metrics from the offline path are reported through the new OfflineMetricCollector to a Codahale MetricRegistry, landing alongside online-query metrics under the same event names.

Note: the stored-query name is not consumed by anything yet at runtime. Names are persisted so that a future feature (e.g., named-plan addressing or plan pinning) can pick them up without another metadata migration.

Worth discussing: offline and online planner metrics currently land under the same event names. That makes the existing dashboards just work but conflates startup pre-warming costs with user-query latencies. We should decide whether to keep them merged, split into distinct events or tag them with a dimension so they can be filtered apart.