[SPARK-29587][SQL] Support SQL Standard type real as float(4) numeric as decimal

### What changes were proposed in this pull request?
The types decimal and numeric are equivalent. Both types are part of the SQL standard.

the real type is  4 bytes, variable-precision, inexact, 6 decimal digits precision, same as our float, part of the SQL standard.

### Why are the changes needed?

improve sql standard support
other dbs
https://www.postgresql.org/docs/9.3/datatype-numeric.html
https://prestodb.io/docs/current/language/types.html#floating-point
http://www.sqlservertutorial.net/sql-server-basics/sql-server-data-types/
MySQL treats REAL as a synonym for DOUBLE PRECISION (a nonstandard variation), unless the REAL_AS_FLOAT SQL mode is enabled.
In MySQL, NUMERIC is implemented as DECIMAL, so the following remarks about DECIMAL apply equally to NUMERIC.

### Does this PR introduce any user-facing change?

no
### How was this patch tested?

add ut

Closes #26537 from yaooqinn/SPARK-29587.

Authored-by: Kent Yao <[email protected]>
Signed-off-by: Wenchen Fan <[email protected]>

Author: yaooqinn

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/parser/AstBuilder.scala

@@ -2179,17 +2179,18 @@ class AstBuilder(conf: SQLConf) extends SqlBaseBaseVisitor[AnyRef] with Logging
       case ("smallint" | "short", Nil) => ShortType
       case ("int" | "integer", Nil) => IntegerType
       case ("bigint" | "long", Nil) => LongType
-      case ("float", Nil) => FloatType
+      case ("float" | "real", Nil) => FloatType
       case ("double", Nil) => DoubleType
       case ("date", Nil) => DateType
       case ("timestamp", Nil) => TimestampType
       case ("string", Nil) => StringType
       case ("character" | "char", length :: Nil) => CharType(length.getText.toInt)
       case ("varchar", length :: Nil) => VarcharType(length.getText.toInt)
       case ("binary", Nil) => BinaryType
-      case ("decimal" | "dec", Nil) => DecimalType.USER_DEFAULT
-      case ("decimal" | "dec", precision :: Nil) => DecimalType(precision.getText.toInt, 0)
-      case ("decimal" | "dec", precision :: scale :: Nil) =>
+      case ("decimal" | "dec" | "numeric", Nil) => DecimalType.USER_DEFAULT
+      case ("decimal" | "dec" | "numeric", precision :: Nil) =>
+        DecimalType(precision.getText.toInt, 0)
+      case ("decimal" | "dec" | "numeric", precision :: scale :: Nil) =>
         DecimalType(precision.getText.toInt, scale.getText.toInt)
       case ("interval", Nil) => CalendarIntervalType
       case (dt, params) =>

sql/core/src/test/resources/sql-tests/inputs/show-create-table.sql

@@ -59,3 +59,8 @@ TBLPROPERTIES ('a' = '1');
 
 SHOW CREATE TABLE tbl;
 DROP TABLE tbl;
+
+-- float alias real and decimal alias numeric
+CREATE TABLE tbl (a REAL, b NUMERIC, c NUMERIC(10), d NUMERIC(10,1)) USING parquet;
+SHOW CREATE TABLE tbl;
+DROP TABLE tbl;

sql/core/src/test/resources/sql-tests/results/show-create-table.sql.out

@@ -1,5 +1,5 @@
 -- Automatically generated by SQLQueryTestSuite
--- Number of queries: 24
+-- Number of queries: 27
 
 
 -- !query 0
@@ -220,3 +220,28 @@ DROP TABLE tbl
 struct<>
 -- !query 23 output
 
+
+
+-- !query 24
+CREATE TABLE tbl (a REAL, b NUMERIC, c NUMERIC(10), d NUMERIC(10,1)) USING parquet
+-- !query 24 schema
+struct<>
+-- !query 24 output
+
+
+
+-- !query 25
+SHOW CREATE TABLE tbl
+-- !query 25 schema
+struct<createtab_stmt:string>
+-- !query 25 output
+CREATE TABLE `tbl` (`a` FLOAT, `b` DECIMAL(10,0), `c` DECIMAL(10,0), `d` DECIMAL(10,1))
+USING parquet
+
+
+-- !query 26
+DROP TABLE tbl
+-- !query 26 schema
+struct<>
+-- !query 26 output
+