Repository: tobymao/sqlglot

Issue:
Druid: CURRENT_TIMESTAMP function being read and transpiled wrong
Hi i noticed this issue when reading and transpiling druid sql.. I have made the relevant changes needed on my local, and would like to make a pr contribution for this.
This bug will cause it to fail on druid side with the error 
`Error: Plan validation failed
org.apache.calcite.runtime.CalciteContextException: From line 1, column 48 to line 1, column 66: No match found for function signature CURRENT_TIMESTAMP()
org.apache.calcite.tools.ValidationException`

**Fully reproducible code snippet**
`
sql_query = "SELECT * FROM \"TABLE\" WHERE  \"__time\" >= CURRENT_TIMESTAMP - INTERVAL '1' DAY"
ast = parse_one(sql_query,read="druid")
print(ast.sql(dialect="druid"))

#Output : "SELECT * FROM \"TABLE\" WHERE  \"__time\" >= CURRENT_TIMESTAMP() - INTERVAL '1' DAY"

#Expected Output: "SELECT * FROM \"TABLE\" WHERE  \"__time\" >= CURRENT_TIMESTAMP - INTERVAL '1' DAY"
`

**Official Documentation**
Please include links to official SQL documentation related to your issue.
https://druid.apache.org/docs/latest/querying/sql-functions/#current_timestamp

Hints from issue comments:

Solution patch:
diff --git a/sqlglot/dialects/druid.py b/sqlglot/dialects/druid.py
index 73e1fdc467..8b95abb572 100644
--- a/sqlglot/dialects/druid.py
+++ b/sqlglot/dialects/druid.py
@@ -15,5 +15,6 @@ class Generator(generator.Generator):
 
         TRANSFORMS = {
             **generator.Generator.TRANSFORMS,
+            exp.CurrentTimestamp: lambda *_: "CURRENT_TIMESTAMP",
             exp.Mod: rename_func("MOD"),
         }


Test patch:
diff --git a/tests/dialects/test_druid.py b/tests/dialects/test_druid.py
index ba634fbef4..90938b9672 100644
--- a/tests/dialects/test_druid.py
+++ b/tests/dialects/test_druid.py
@@ -13,7 +13,8 @@ def test_druid(self):
         self.validate_identity("SELECT FLOOR(__time TO WEEK) FROM t")
         self.validate_identity("SELECT FLOOR(col) FROM t")
         self.validate_identity("SELECT FLOOR(price, 2) AS rounded_price FROM t")
-
+        self.validate_identity("SELECT CURRENT_TIMESTAMP")
+        
         # validate across all dialects
         write = {dialect.value: "FLOOR(__time TO WEEK)" for dialect in Dialects}
         self.validate_all(