### What changes were proposed in this pull request?
This PR add unlimited MATCHED and NOT MATCHED clauses in MERGE INTO statement.
### Why are the changes needed?
Now the MERGE INTO syntax is,
```
MERGE INTO [db_name.]target_table [AS target_alias]
USING [db_name.]source_table [<time_travel_version>] [AS source_alias]
ON <merge_condition>
[ WHEN MATCHED [ AND <condition> ] THEN <matched_action> ]
[ WHEN MATCHED [ AND <condition> ] THEN <matched_action> ]
[ WHEN NOT MATCHED [ AND <condition> ] THEN <not_matched_action> ]
```
It would be nice if we support unlimited MATCHED and NOT MATCHED clauses in MERGE INTO statement, because users may want to deal with different "AND <condition>"s, the result of which just like a series of "CASE WHEN"s. The expected syntax looks like
```
MERGE INTO [db_name.]target_table [AS target_alias]
USING [db_name.]source_table [<time_travel_version>] [AS source_alias]
ON <merge_condition>
[when_matched_clause [, ...]]
[when_not_matched_clause [, ...]]
```
where when_matched_clause is
```
WHEN MATCHED [ AND <condition> ] THEN <matched_action>
```
and when_not_matched_clause is
```
WHEN NOT MATCHED [ AND <condition> ] THEN <not_matched_action>
```
matched_action can be one of
```
DELETE
UPDATE SET * or
UPDATE SET col1 = value1 [, col2 = value2, ...]
```
and not_matched_action can be one of
```
INSERT *
INSERT (col1 [, col2, ...]) VALUES (value1 [, value2, ...])
```
### Does this PR introduce _any_ user-facing change?
Yes. The SQL command changes, but it is backward compatible.
### How was this patch tested?
New tests added.
Closes#28875 from xianyinxin/SPARK-32030.
Authored-by: xy_xin <xianyin.xxy@alibaba-inc.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
20cd47e82d