### What changes were proposed in this pull request?
Add and register three new functions: `TIMESTAMP_SECONDS`, `TIMESTAMP_MILLIS` and `TIMESTAMP_MICROS`
A test is added.
Reference: [BigQuery](https://cloud.google.com/bigquery/docs/reference/standard-sql/timestamp_functions?hl=en#timestamp_seconds)
### Why are the changes needed?
People will have convenient way to get timestamps from seconds,milliseconds and microseconds.
### Does this PR introduce _any_ user-facing change?
Yes, people will have the following ways to get timestamp:
```scala
sql("select TIMESTAMP_SECONDS(t.a) as timestamp from values(1230219000),(-1230219000) as t(a)").show(false)
```
```
+-------------------------+
|timestamp |
+-------------------------+
|2008-12-25 23:30:00|
|1931-01-07 16:30:00|
+-------------------------+
```
```scala
sql("select TIMESTAMP_MILLIS(t.a) as timestamp from values(1230219000123),(-1230219000123) as t(a)").show(false)
```
```
+-------------------------------+
|timestamp |
+-------------------------------+
|2008-12-25 23:30:00.123|
|1931-01-07 16:29:59.877|
+-------------------------------+
```
```scala
sql("select TIMESTAMP_MICROS(t.a) as timestamp from values(1230219000123123),(-1230219000123123) as t(a)").show(false)
```
```
+------------------------------------+
|timestamp |
+------------------------------------+
|2008-12-25 23:30:00.123123|
|1931-01-07 16:29:59.876877|
+------------------------------------+
```
### How was this patch tested?
Unit test.
Closes#28534 from TJX2014/master-SPARK-31710.
Authored-by: TJX2014 <xiaoxingstack@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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