## What changes were proposed in this pull request?
This pr adds transform_keys function which applies the function to each entry of the map and transforms the keys.
```javascript
> SELECT transform_keys(map(array(1, 2, 3), array(1, 2, 3)), (k,v) -> k + 1);
map(2->1, 3->2, 4->3)
> SELECT transform_keys(map(array(1, 2, 3), array(1, 2, 3)), (k,v) -> k + v);
map(2->1, 4->2, 6->3)
```
## How was this patch tested?
Added tests.
Closes#22013 from codeatri/SPARK-23939.
Authored-by: codeatri <nehapatil6@gmail.com>
Signed-off-by: Takuya UESHIN <ueshin@databricks.com>
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