Core: Read REST response headers without regard to case

[moomindani]

Closes #17978.

Freshness-aware table loading did nothing at all when the catalog server wrote the ETag response header in any spelling other than ETag. HTTPClient collected response headers into a map keyed by the name exactly as received, RESTSessionCatalog copied that into another HashMap and read the tag back as HttpHeaders.ETAG, so a server that sent etag left tableCache empty. No later load sent If-None-Match, and nothing surfaced the fact that the server had answered with a tag.

Header field names are case-insensitive (RFC 9110 section 5.1), and lowercase is mandatory in HTTP/2 (RFC 9113 section 8.2.1), so a client cannot key on the spelling a server happened to choose. Both maps are now case-insensitive, which fixes the lookup without changing what callers see otherwise.

I hit this against an Armeria-based catalog server, which writes HTTP/1 header names in their lowercase HTTP/2 form by default: the server returned a correct ETag for a conditional loadTable and the client could not see it. The same applies to any server reached over HTTP/2.

Tests:

• TestHTTPClient.responseHeadersAreReadWithoutRegardToCase has the mock server answer with etag and asserts the map handed to the caller resolves both spellings, covering the collection point.
• TestFreshnessAwareLoading.freshnessAwareLoadingWithLowercaseETagHeader drives a catalog through an adapter that reports its response headers lowercased, and asserts the table cache is still populated and the second load is answered from it, covering the consumer.
• Both fail on an unfixed tree. ./gradlew :iceberg-core:test --tests "org.apache.iceberg.rest.*" passes.

[uros-b]

Looks like a correct, minimal, RFC 9110-grounded fix for a real HTTP/2 and lowercase-header-normalization interop bug: both collection points must be case-insensitive for the ETag round-trip to work, both edits are present, and both new tests trace the full path and fail on an unfixed tree. The sole red CI check is an unrelated Gradle-wrapper infra flake. Otherwise, should be good to go! Thank you @moomindani.