[FLINK-40357][table] Add ARRAY_FLATTEN function to Flink SQL

[cytq-123]

What is the purpose of the change

This PR adds the ARRAY_FLATTEN function to Flink SQL, which flattens a nested array by one level.

Brief change log

• Add ARRAY_FLATTEN function definition in BuiltInFunctionDefinitions
• Implement ArrayFlattenFunction runtime class
• Add type inference strategy in SpecificTypeStrategies
• Add comprehensive test cases in CollectionFunctionsITCase

Verifying this change

This change added tests and can be verified as follows:

• Added 10 test cases in CollectionFunctionsITCase#arrayFlattenTestCases()
• Tests cover basic functionality, NULL handling, and edge cases

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

• Dependencies: no
• The public API: no
• The serializers: no
• The runtime per-record code paths: no
• 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? JavaDocs

[flinkbot]

CI report:

• 61dd151 Azure: SUCCESS

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