[FLINK-37926][table] Support casting from VARIANT to MAP

[raminqaf]

What is the purpose of the change

This pull request adds CAST and TRY_CAST from VARIANT to MAP<STRING, V>. It is the last of the stacked constructed-cast PRs and builds on the same design: ARRAY (merged), ROW/STRUCTURED (#29092), and now MAP. It is stacked on #29092 and should be reviewed and merged after it.

MAP is the schemaless read of an object: unlike ROW, no field is named in advance, so it is the way to read an object whose keys are dynamic or unknown at query time.

Key semantics:

• The variant must be an object. Each field name becomes a key and each value casts to V by the same VARIANT-to-value rules, recursively.
• The key type must be a character string, since a variant object's keys are always strings. A non-string key type is rejected at validation.
• A value present but set to a variant null maps to SQL NULL when V is nullable and fails the cast when V is NOT NULL. An empty object casts to an empty map.
• A MAP<STRING, VARIANT> value type is the identity, so a variant null is kept as a variant null rather than downgraded to SQL NULL. This matches ARRAY<VARIANT> and a ROW with VARIANT fields.
• Leaves are never parsed, and TRY_CAST returns NULL for the entire value on any failure.

Brief change log

• Add VariantToMapCastRule that casts a variant object to MAP<STRING, V>, mapping field names to keys and casting each value with the existing VARIANT-to-value rules.
• Allow the explicit cast in LogicalTypeCasts when the key is a character string and VARIANT casts to the value type.
• Document the VARIANT to MAP cast and mark VARIANT → MAP in the cast-support matrix.

Verifying this change

This change added tests and can be verified as follows:

• CastRulesTest: key/value mapping, an empty object, a non-object input, and MAP<STRING, VARIANT> with and without a variant null value.
• CastFunctionITCase: end-to-end SQL and Table API cases (the Table API cases use the parseJson() expression method).
• LogicalTypeCastsTest and CastRuleProviderTest: castability and rule resolution, including the non-string-key rejection.

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

• Dependencies (does it add or upgrade a dependency): no
• The public API, i.e., is any changed class annotated with @Public(Evolving): no
• The serializers: no
• The runtime per-record code paths (performance sensitive): no
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: 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

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CI report:

• 0c706e9 Azure: SUCCESS

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[twalthr]

LGTM, thanks @raminqaf