Validate enum values when requested

[fallintoplace]

What changed

• Validate numeric enum values when checkValue is true in Append() and SetAt().
• Throw ValidationError when the value is not defined in the enum type.
• Keep the default unchecked behavior unchanged.

Tests

• cmake --build build --target clickhouse-cpp-ut --parallel 8
• ./build/ut/clickhouse-cpp-ut --gtest_filter='ColumnsCase.Enum*'