[SPARK-50692] Add RPAD pushdown support

### What changes were proposed in this pull request?
This PR makes RPAD and LPAD functions translatable across connectors. Individual connectors can now support them via their dialects.

### Why are the changes needed?
When trying to compare CHAR(...) and a literal, Spark injects a RPAD function to make the CHAR parameter the same length. This behaviour is per ANSI standard.

The problem is that RPAD is not only an unsupported, but an untranslatable function. This prevents CHAR + literal comparison pushdown.

Solution is to support RPAD pushdowns and implement it when needed.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
This PR only introduces the ability to support the RPAD pushdown. Individual connectors will have their own tests.

### Was this patch authored or co-authored using generative AI tooling?
No

Closes #49325 from andrej-db/SPARK-50692-RPAD.

Authored-by: andrej-gobeljic_data <andrej.gobeljic@databricks.com>
Signed-off-by: Wenchen Fan <wenchen@databricks.com>

sql/catalyst/src/main/java/org/apache/spark/sql/connector/util/V2ExpressionSQLBuilder.java

@@ -118,7 +118,7 @@ yield visitBinaryArithmetic(
           "COT", "ASIN", "ASINH", "ACOS", "ACOSH", "ATAN", "ATANH", "ATAN2", "CBRT", "DEGREES",
           "RADIANS", "SIGN", "WIDTH_BUCKET", "SUBSTRING", "UPPER", "LOWER", "TRANSLATE",
           "DATE_ADD", "DATE_DIFF", "TRUNC", "AES_ENCRYPT", "AES_DECRYPT", "SHA1", "SHA2", "MD5",
-          "CRC32", "BIT_LENGTH", "CHAR_LENGTH", "CONCAT" ->
+          "CRC32", "BIT_LENGTH", "CHAR_LENGTH", "CONCAT", "RPAD", "LPAD" ->
           visitSQLFunction(name, expressionsToStringArray(e.children()));
         case "CASE_WHEN" -> visitCaseWhen(expressionsToStringArray(e.children()));
         case "TRIM" -> visitTrim("BOTH", expressionsToStringArray(e.children()));

sql/core/src/main/scala/org/apache/spark/sql/catalyst/util/V2ExpressionBuilder.scala

@@ -305,6 +305,8 @@ class V2ExpressionBuilder(e: Expression, isPredicate: Boolean = false) extends L
     case _: Md5 => generateExpressionWithName("MD5", expr, isPredicate)
     case _: Sha1 => generateExpressionWithName("SHA1", expr, isPredicate)
     case _: Sha2 => generateExpressionWithName("SHA2", expr, isPredicate)
+    case _: StringLPad => generateExpressionWithName("LPAD", expr, isPredicate)
+    case _: StringRPad => generateExpressionWithName("RPAD", expr, isPredicate)
     // TODO supports other expressions
     case ApplyFunctionExpression(function, children) =>
       val childrenExpressions = children.flatMap(generateExpression(_))