## What changes were proposed in this pull request?
Currently `PartitioningAwareFileIndex` accepts an optional parameter `userPartitionSchema`. If provided, it will combine the inferred partition schema with the parameter.
However,
1. to get `userPartitionSchema`, we need to combine inferred partition schema with `userSpecifiedSchema`
2. to get the inferred partition schema, we have to create a temporary file index.
Only after that, a final version of `PartitioningAwareFileIndex` can be created.
This can be improved by passing `userSpecifiedSchema` to `PartitioningAwareFileIndex`.
With the improvement, we can reduce redundant code and avoid parsing the file partition twice.
## How was this patch tested?
Unit test
Author: Gengliang Wang <gengliang.wang@databricks.com>
Closes#21004 from gengliangwang/PartitioningAwareFileIndex.
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