feat: Introduce convert Expr to SQL string API and basic feature (#9517)

* feat: convert Expr to SQL string

* fix: add license headers

* fix: make Unparser and Dialect public

* Update datafusion/sql/src/unparser/dialect.rs

* fmt

---------

Co-authored-by: Andrew Lamb <andrew@nerdnetworks.org>

datafusion/sql/src/lib.rs

@@ -36,6 +36,7 @@ mod relation;
 mod select;
 mod set_expr;
 mod statement;
+pub mod unparser;
 pub mod utils;
 mod values;
 

datafusion/sql/src/unparser/dialect.rs

@@ -0,0 +1,73 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+/// Dialect is used to capture dialect specific syntax.
+/// Note: this trait will eventually be replaced by the Dialect in the SQLparser package
+///
+/// See <https://github.com/sqlparser-rs/sqlparser-rs/pull/1170>
+pub trait Dialect {
+    fn identifier_quote_style(&self) -> Option<char>;
+}
+pub struct DefaultDialect {}
+
+impl Dialect for DefaultDialect {
+    fn identifier_quote_style(&self) -> Option<char> {
+        None
+    }
+}
+
+pub struct PostgreSqlDialect {}
+
+impl Dialect for PostgreSqlDialect {
+    fn identifier_quote_style(&self) -> Option<char> {
+        Some('"')
+    }
+}
+
+pub struct MySqlDialect {}
+
+impl Dialect for MySqlDialect {
+    fn identifier_quote_style(&self) -> Option<char> {
+        Some('`')
+    }
+}
+
+pub struct SqliteDialect {}
+
+impl Dialect for SqliteDialect {
+    fn identifier_quote_style(&self) -> Option<char> {
+        Some('`')
+    }
+}
+
+pub struct CustomDialect {
+    identifier_quote_style: Option<char>,
+}
+
+impl CustomDialect {
+    pub fn new(identifier_quote_style: Option<char>) -> Self {
+        Self {
+            identifier_quote_style,
+        }
+    }
+}
+
+impl Dialect for CustomDialect {
+    fn identifier_quote_style(&self) -> Option<char> {
+        self.identifier_quote_style
+    }
+}