LLVM JIT function would support append_slice and encode columns with input row

Now we support parameter append slice to append input row columns to window output columns. We need to support generate single row with append row columns at one time.

The issue mentions LLVM JIT function support for append_slice and encode columns with input row. The image suggests a code generation diagram, but specifics are missing. Research the OpenMLDB codebase for window function handling and LLVM JIT code generation. Look for existing append_slice implementations and window output column generation. Determine how to modify the JIT to produce a single row with appended columns.

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