Refactor `SQLRequestRowBatch`

There are two sub-tasks:
- `SQLRequestRowBatch` copies string multiple times in the `AddRow` function. It's better to use `RefCountedSlice` rather than `String`.
- Extract a separate encode function from `RowSelector::Select`

Look for the SQLRequestRowBatch class and its AddRow function to understand the string copying issue. Investigate RefCountedSlice usage elsewhere in the codebase. Also examine RowSelector::Select to identify the encode logic to extract. Start by building and running existing tests related to these components to ensure changes don't break functionality.

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