Enable AT/BEFORE timestamp versioning syntax in OpteryxDialect

Add supports_timestamp_versioning() returning true to enable:
- AT(TIMESTAMP => '...') syntax (Snowflake-style)
- BEFORE(TIMESTAMP => '...') syntax
- FOR SYSTEM_TIME AS OF '...' syntax (SQL:2011 standard)

This allows the parser to recognize these time-travel syntaxes
even though Opteryx doesn't use them yet.

Co-authored-by: joocer <1688479+joocer@users.noreply.github.com>

Author: Copilot

src/opteryx_dialect.rs

@@ -126,4 +126,13 @@ impl Dialect for OpteryxDialect {
         true
     }
 
+    /// Returns true if the dialect supports timestamp versioning for time-travel queries.
+    /// This enables syntax like:
+    /// - `SELECT * FROM table AT(TIMESTAMP => '2024-12-15 00:00:00')`
+    /// - `SELECT * FROM table BEFORE(TIMESTAMP => '2024-12-15 00:00:00')`
+    /// - `SELECT * FROM table FOR SYSTEM_TIME AS OF '2024-12-15 00:00:00'`
+    fn supports_timestamp_versioning(&self) -> bool {
+        true
+    }
+
 }