RestCatalog: support freshness-aware table loading (If-None-Match / 304 Not Modified), as Iceberg Java does #4075

Feature Request / Improvement

Iceberg Java already does this. Its REST client loads tables "freshness-aware": it keeps a table cache, sends If-None-Match with the cached ETag, and reuses the cached table when the server answers 304 Not Modified. The cache is configured by two catalog properties in RESTCatalogProperties:

• rest-table-cache.max-entries (default 100)
• rest-table-cache.expire-after-write-ms (default 5 minutes)

This asks for the same in pyiceberg's RestCatalog, whose load_table today always performs a full load.

What the spec defines (open-api/rest-catalog-open-api.yaml):

• loadTable (GET /v1/{prefix}/namespaces/{namespace}/tables/{table}) accepts an optional If-None-Match header: "allows the server to return 304 (Not Modified) if the metadata is current. The content is the value of the ETag received in a CreateTableResponse, LoadTableResponse or CommitTableResponse."
• LoadTableResponse carries an etag header.
• 304: "Not Modified - Based on the content of the 'If-None-Match' header the table metadata has not changed since."

What pyiceberg does today (0.12.0 and main): RestCatalog.load_table sends no If-None-Match, keeps no ETag from load/create/commit responses, and has no handling for a 304.

Use case: clients that load the same tables repeatedly — Kubernetes operators reconciling on an interval, long-running services, notebooks re-reading tables — download and parse the full metadata JSON on every load_table, even when nothing changed. That JSON grows with snapshot history and can be large for busy tables. Servers already answer conditionally: Lakekeeper sends an ETag and replies 304 from v0.11.0.

Suggested shape (matching Iceberg Java, so the same catalog properties work across clients):

• an optional, bounded, per-catalog cache of identifier -> (ETag, loaded metadata and metadata location), filled from the etag header of load, create and commit responses;
• when an entry exists, load_table sends If-None-Match and, on 304, returns the cached table instead of treating the empty body as an error;
• configured by Java's property names, rest-table-cache.max-entries and rest-table-cache.expire-after-write-ms.