### What changes were proposed in this pull request?
1/ conv() have inconsistency in behavior where the returned value is different above the 64 char threshold.
```
scala> spark.sql("select conv(repeat('?', 64), 10, 16)").show
+---------------------------+
|conv(repeat(?, 64), 10, 16)|
+---------------------------+
| 0|
+---------------------------+
scala> spark.sql("select conv(repeat('?', 65), 10, 16)").show // which should be 0
+---------------------------+
|conv(repeat(?, 65), 10, 16)|
+---------------------------+
| FFFFFFFFFFFFFFFF|
+---------------------------+
scala> spark.sql("select conv(repeat('?', 65), 10, -16)").show // which should be 0
+----------------------------+
|conv(repeat(?, 65), 10, -16)|
+----------------------------+
| -1|
+----------------------------+
scala> spark.sql("select conv(repeat('?', 64), 10, -16)").show
+----------------------------+
|conv(repeat(?, 64), 10, -16)|
+----------------------------+
| 0|
+----------------------------+
```
2/ conv should return result equal to max unsigned long value in base toBase when there is overflow
```
scala> spark.sql(select conv('aaaaaaa0aaaaaaa0a', 16, 10)).show // which should be 18446744073709551615
+-------------------------------+
|conv(aaaaaaa0aaaaaaa0a, 16, 10)|
+-------------------------------+
| 12297828695278266890|
+-------------------------------+
```
### Why are the changes needed?
Bug fix, this pull request aim to make conv function behave similarly with the behavior of conv function from MySQL database
### Does this PR introduce _any_ user-facing change?
change in result of conv() function
### How was this patch tested?
add test
Closes#33459 from dgd-contributor/SPARK-36229_convInconsistencyBehaviorWithMoreThan64Characters.
Authored-by: dgd-contributor <dgd_contributor@viettel.com.vn>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>
e1c50ff779