[GH-3176] Add Geography support for convex hull and collection

[jiayuasu]

Did you read the Contributor Guide?

• Yes, I have read the Contributor Rules and Contributor Development Guide

Is this PR related to a ticket?

• Yes, and the PR name follows the format [GH-XXX] my subject. Closes Add Geography support for ST_ConvexHull and the geometry-collecting aggregate #3176

What changes were proposed in this PR?

• Add spherical Geography support for ST_ConvexHull, including geodesic edges, dimensionality preservation, antimeridian handling, and explicit full-sphere rejection.
• Extend scalar ST_Collect and aggregate ST_Collect_Agg to preserve either the Geometry or Geography logical type.
• Add the Geography overloads to both Spark and Flink, including Flink's ST_Collect_Aggr alias.
• Preserve null handling, duplicates, SRIDs, empty values, and aggregate merge behavior.
• Correct empty Geography LineString WKB serialization and normalize the legacy header-only representation.

How was this patch tested?

• Common Geography and WKB tests: 111 passed.
• Spark Geography and SRID suites: 80 passed.
• Spark aggregate suite: 30 passed.
• Spark DataFrame aggregate API test: 1 passed.
• Flink Geography, aggregate, merge, and Geometry regression tests: 39 passed.
• Spark 4.1 with Scala 2.13 compile: passed.
• Spotless, pre-commit hooks, and git diff --check: passed.

Did this PR include necessary documentation updates?

• Yes, I am adding new Geography APIs using the current SNAPSHOT version number, v1.9.1.
• Yes, I updated the Spark SQL and Flink API pages and function indexes.