[Feature Request]: Implement MySQL Encode()/Decode() function

[charleschile]

User description

What type of PR is this?

• [ ] API-change
• [ ] BUG
• [ ] Improvement
• [ ] Documentation
• [x] Feature
• [ ] Test and CI
• [ ] Code Refactoring

Which issue(s) this PR fixes:

issue #15027

What this PR does / why we need it:

implement Encode() and Decode() functions

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PR Type

Enhancement, Tests

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Description

• Implemented Encode and Decode functions for string encryption and decryption.
• Added helper structs and methods to support encoding and decoding logic.
• Registered Encode and Decode functions in the built-in function list.
• Added unit tests for Encode and Decode functions, covering various scenarios.
• Added SQL test cases and expected results for Encode and Decode functions.

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Changes walkthrough 📝

	Relevant files
Enhancement	func_unary.go Implement Encode and Decode functions for string encryption pkg/sql/plan/function/func_unary.go Added Encode and Decode functions for string encryption and decryption. Introduced helper structs and methods for encoding and decoding logic. Implemented encodeToBytes and decodeFromBytes helper functions. +149/-0  list_builtIn.go Register Encode and Decode functions in built-in list        pkg/sql/plan/function/list_builtIn.go Registered Encode and Decode functions in the built-in function list. +42/-0
Tests	func_unary_test.go Add unit tests for Encode and Decode functions                      pkg/sql/plan/function/func_unary_test.go Added unit tests for Encode and Decode functions. Introduced test cases for various encoding and decoding scenarios. +167/-0  func_decode_encode.result Add expected results for Encode and Decode tests                  test/distributed/cases/function/func_decode_encode.result Added expected results for Encode and Decode function tests. +33/-0    func_decode_encode.sql Add SQL test cases for Encode and Decode functions              test/distributed/cases/function/func_decode_encode.sql Added SQL test cases for Encode and Decode functions. +11/-0

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