[api] Configure keep-alive timeout for HTTP BLOB descriptors

[wwj6591812]

Purpose

Add a table option to cap how long HTTP connections used by descriptor-backed BLOB reads remain eligible for reuse after a response completes.

blob-descriptor.http.keep-alive-timeout is unset by default, so existing behavior remains unchanged. This option is an idle connection reuse cap; it is not a connect, socket, or response-body timeout.

Changes

• Propagate the option through Flink dynamic sink options, serialized URI reader factories, and source-table descriptors.
• Use the shorter of the server-advertised keep-alive duration and the configured client cap.
• Apply the cap to HEAD probes, initial GET requests, range resume requests, and replay requests.
• Isolate explicitly configured BLOB reads in a dedicated connection pool so the option does not affect REST Catalog traffic or unconfigured reads.
• Preserve the existing FileIO context and credentials for non-HTTP descriptors.
• Document the target-table OPTIONS hint.

Relationship to #9271

#9271 recovers response bodies that are truncated while an active GET is being consumed. This change addresses a different boundary: it limits reuse of idle pooled connections after a response has completed. The two mechanisms are complementary; configuring this option does not replace body-truncation recovery.

Scope and performance

Unconfigured reads keep using the existing shared client and behavior. Configured BLOB reads use a separate bounded pool (100 total/per route). Before leasing a configured connection, idle entries older than the requested cap are removed; a smaller cap can reduce connection reuse but does not interrupt leased or active response streams.

Tests

• HttpClientUtilsTest (39 tests)
• UriReaderFactoryTest and BlobDescriptorUtilsTest (20 tests)
• CoreOptionsTest and BlobDescriptorReaderFactoryTest (18 tests)
• BlobTableITCase#testWriteBlobWithHttpUrlDescriptor
• ConfigOptionsDocsCompletenessITCase
• Spotless and git diff --check

API and format

This adds one optional table configuration key. It does not change the BLOB descriptor or table storage format.

[wwj6591812]

@JingsongLi hi, please cc, thx