### What changes were proposed in this pull request?
We now have two different implementation for multi-units interval strings to CalendarInterval type values.
One is used to covert interval string literals to CalendarInterval. This approach will re-delegate the interval string to spark parser which handles the string as a `singleInterval` -> `multiUnitsInterval` -> eventually call `IntervalUtils.fromUnitStrings`
The other is used in `Cast`, which eventually calls `IntervalUtils.stringToInterval`. This approach is ~10 times faster than the other.
We should unify these two for better performance and simple logic. this pr uses the 2nd approach.
### Why are the changes needed?
We should unify these two for better performance and simple logic.
### Does this PR introduce any user-facing change?
no
### How was this patch tested?
we shall not fail on existing uts
Closes#26491 from yaooqinn/SPARK-29870.
Authored-by: Kent Yao <yaooqinn@hotmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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