feat: support freshness-aware table loading in RestCatalog

[YuangGao]

Closes #4075

Rationale for this change

• load_table never sent If-None-Match, and a body-less 304 failed to parse.
• Tables loaded with an ETag are now cached; a later load sends If-None-Match and returns a fresh Table from the cached response on 304.

Are these changes tested?

• test_rest.py — reuse on 304, 304 without a cached table, and max-entries=0.
• test_rest_catalog.py — 200 then 304 against the REST fixture.

Are there any user-facing changes?

• New catalog properties rest-table-cache.max-entries (default 100, 0 disables) and rest-table-cache.expire-after-write-ms (default 5 minutes).