[GH-3179] Add Geography support for ST_X and ST_Y

[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_X and ST_Y #3179

What changes were proposed in this PR?

• Add Geography overloads for ST_X and ST_Y.
• Dispatch Spark expressions to the Geometry or Geography implementation based on the input type.
• Return NULL for non-point and empty Geography inputs, matching the existing Geometry behavior.

How was this patch tested?

• Common Geography function tests: 77 passed.
• Spark Geography function tests: 50 passed.
• Existing Spark Geometry function regression suite: 235 passed.

Did this PR include necessary documentation updates?

• Yes, I am adding a new API using the current SNAPSHOT version number, v1.9.1.
• Yes, I have updated the Geography function index and added pages for ST_X and ST_Y.