### What changes were proposed in this pull request?
This PR proposes to propagate the name used for registering UDFs to `ScalaUDF`, `ScalaUDAF` and `ScaalAggregator`.
Note that `PythonUDF` gets the name correctly: 466c045bfa/python/pyspark/sql/udf.py (L358-L359)
, and same for Hive UDFs:
466c045bfa/sql/hive/src/main/scala/org/apache/spark/sql/hive/HiveSessionCatalog.scala (L67)
### Why are the changes needed?
This PR can help in the following scenarios:
1) Better EXPLAIN output
2) By adding `def name: String` to `UserDefinedExpression`, we can match an expression by `UserDefinedExpression` and look up the catalog, an use case needed for #31273.
### Does this PR introduce _any_ user-facing change?
The EXPLAIN output involving udfs will be changed to use the name used for UDF registration.
For example, for the following:
```
sql("CREATE TEMPORARY FUNCTION test_udf AS 'org.apache.spark.examples.sql.Spark33084'")
sql("SELECT test_udf(col1) FROM VALUES (1), (2), (3)").explain(true)
```
The output of the optimized plan will change from:
```
Aggregate [spark33084(cast(col1#223 as bigint), org.apache.spark.examples.sql.Spark330846906be0f, 1, 1) AS spark33084(col1)#237]
+- LocalRelation [col1#223]
```
to
```
Aggregate [test_udf(cast(col1#223 as bigint), org.apache.spark.examples.sql.Spark330847a62d697, 1, 1, Some(test_udf)) AS test_udf(col1)#237]
+- LocalRelation [col1#223]
```
### How was this patch tested?
Added new tests.
Closes#31500 from imback82/udaf_name.
Authored-by: Terry Kim <yuminkim@gmail.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
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