What is the purpose of the change

This pull request adds SQL constructor and accessor functions for the GEOGRAPHY
logical type introduced in #28740.

The functions provide conversion between GEOGRAPHY values and their WKT/WKB
representations. This PR is intended to be merged after #28740.

Jira: https://issues.apache.org/jira/browse/FLINK-39904
Discussion: https://lists.apache.org/thread/flbj8dfdfgs26klrxt7xch3r9785ky67

Brief change log

• Added ST_GEOGFROMTEXT and ST_GEOGFROMWKB constructors for creating
  GEOGRAPHY values from WKT and WKB.
• Added ST_ASTEXT and ST_ASWKB accessors for converting GEOGRAPHY values
  to WKT and WKB.
• Added planner registrations and runtime implementations for the functions.
• Added planner integration tests and runtime unit tests.

Verifying this change

This change added tests and can be verified as follows:

• Added planner integration tests for WKT/WKB constructor and accessor functions.
• Added runtime unit tests for valid values and invalid WKT/WKB input.

Does this pull request potentially affect one of the following parts:

• Dependencies: no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths: yes
• Anything that affects deployment or recovery: no
• The S3 file system connector: no

Documentation

• Does this pull request introduce a new feature? yes
• If yes, how is the feature documented? docs

Follow-up changes

The following parts are intentionally left out of this PR:

• PyFlink exposure for GEOGRAPHY schemas and functions.
• Format-specific support, including Parquet integration.
• Additional geospatial predicates and measurement functions beyond WKT/WKB
  constructor and accessor support.