## What changes were proposed in this pull request?
uniVocity parser allows to specify only required column names or indexes for [parsing](https://www.univocity.com/pages/parsers-tutorial) like:
```
// Here we select only the columns by their indexes.
// The parser just skips the values in other columns
parserSettings.selectIndexes(4, 0, 1);
CsvParser parser = new CsvParser(parserSettings);
```
In this PR, I propose to extract indexes from required schema and pass them into the CSV parser. Benchmarks show the following improvements in parsing of 1000 columns:
```
Select 100 columns out of 1000: x1.76
Select 1 column out of 1000: x2
```
**Note**: Comparing to current implementation, the changes can return different result for malformed rows in the `DROPMALFORMED` and `FAILFAST` modes if only subset of all columns is requested. To have previous behavior, set `spark.sql.csv.parser.columnPruning.enabled` to `false`.
## How was this patch tested?
It was tested by new test which selects 3 columns out of 15, by existing tests and by new benchmarks.
Author: Maxim Gekk <maxim.gekk@databricks.com>
Author: Maxim Gekk <max.gekk@gmail.com>
Closes#21415 from MaxGekk/csv-column-pruning2.
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