### What changes were proposed in this pull request?
In the PR, I propose to pass the `Pattern.CASE_INSENSITIVE` flag while compiling interval patterns in `CalendarInterval`. This makes casting string values to intervals case insensitive and tolerant to case of the `interval`, `year(s)`, `month(s)`, `week(s)`, `day(s)`, `hour(s)`, `minute(s)`, `second(s)`, `millisecond(s)` and `microsecond(s)`.
### Why are the changes needed?
There are at least 2 reasons:
- To maintain feature parity with PostgreSQL which is not sensitive to case:
```sql
# select cast('10 Days' as INTERVAL);
interval
----------
10 days
(1 row)
```
- Spark is tolerant to case of interval literals. Case insensitivity in casting should be convenient for Spark users.
```sql
spark-sql> SELECT INTERVAL 1 YEAR 1 WEEK;
interval 1 years 1 weeks
```
### Does this PR introduce any user-facing change?
Yes, current implementation produces `NULL` for `interval`, `year`, ... `microsecond` that are not in lower case.
Before:
```sql
spark-sql> SELECT CAST('INTERVAL 10 DAYS' as INTERVAL);
NULL
```
After:
```sql
spark-sql> SELECT CAST('INTERVAL 10 DAYS' as INTERVAL);
interval 1 weeks 3 days
```
### How was this patch tested?
- by new tests in `CalendarIntervalSuite.java`
- new test in `CastSuite`
Closes#26010 from MaxGekk/interval-case-insensitive.
Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>
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