[SPARK-29200][SQL] Optimize extract/date_part for epoch

### What changes were proposed in this pull request?

Refactoring of the `DateTimeUtils.getEpoch()` function by avoiding decimal operations that are pretty expensive, and converting the final result to the decimal type at the end.

### Why are the changes needed?
The changes improve performance of the `getEpoch()` method at least up to **20 times**.
Before:
```
Invoke extract for timestamp:             Best Time(ms)   Avg Time(ms)   Stdev(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------------------------------
cast to timestamp                                   256            277          33         39.0          25.6       1.0X
EPOCH of timestamp                                23455          23550         131          0.4        2345.5       0.0X
```
After:
```
Invoke extract for timestamp:             Best Time(ms)   Avg Time(ms)   Stdev(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------------------------------
cast to timestamp                                   255            294          34         39.2          25.5       1.0X
EPOCH of timestamp                                 1049           1054           9          9.5         104.9       0.2X
```

### Does this PR introduce any user-facing change?
No

### How was this patch tested?

By existing test from `DateExpressionSuite`.

Closes #25881 from MaxGekk/optimize-extract-epoch.

Authored-by: Maxim Gekk <max.gekk@gmail.com>
Signed-off-by: HyukjinKwon <gurwls223@apache.org>

Author: MaxGekk

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/util/DateTimeUtils.scala

@@ -847,9 +847,10 @@ object DateTimeUtils {
    * since 1970-01-01 00:00:00 local time.
    */
   def getEpoch(timestamp: SQLTimestamp, zoneId: ZoneId): Decimal = {
-    val offset = zoneId.getRules.getOffset(microsToInstant(timestamp)).getTotalSeconds
-    val sinceEpoch = BigDecimal(timestamp) / MICROS_PER_SECOND + offset
-    new Decimal().set(sinceEpoch, 20, 6)
+    val offset = SECONDS.toMicros(
+      zoneId.getRules.getOffset(microsToInstant(timestamp)).getTotalSeconds)
+    val sinceEpoch = timestamp + offset
+    Decimal(sinceEpoch, 20, 6)
   }
 
   def currentTimestamp(): SQLTimestamp = instantToMicros(Instant.now())