Avoid clippy warning on reference to static mut

It will be a hard error in 2024 edition so implement it "properly"

warning: creating a mutable reference to mutable static is discouraged
   --> diesel/src/sqlite/connection/mod.rs:229:18
    |
229 |         unsafe { &mut SQLITE_METADATA_LOOKUP }
    |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^ mutable reference to mutable static
    |
    = note: for more information, see issue #114447 <rust-lang/rust#114447>
    = note: this will be a hard error in the 2024 edition
    = note: this mutable reference has lifetime `'static`, but if the static gets accessed (read or written) by any other means, or any other reference is created, then any further use of this mutable reference is Undefined Behavior
    = note: `#[warn(static_mut_refs)]` on by default

diesel/src/sqlite/connection/mod.rs

@@ -124,6 +124,9 @@ pub struct SqliteConnection {
     statement_cache: StatementCache<Sqlite, Statement>,
     raw_connection: RawConnection,
     transaction_state: AnsiTransactionManager,
+    // this exists for the sole purpose of implementing `WithMetadataLookup` trait
+    // and avoiding static mut which will be deprecated in 2024 edition
+    metadata_lookup: (),
     instrumentation: Option<Box<dyn Instrumentation>>,
 }
 
@@ -221,12 +224,9 @@ impl LoadConnection<DefaultLoadingMode> for SqliteConnection {
     }
 }
 
-static mut SQLITE_METADATA_LOOKUP: () = ();
-
 impl WithMetadataLookup for SqliteConnection {
     fn metadata_lookup(&mut self) -> &mut <Sqlite as TypeMetadata>::MetadataLookup {
-        // safe since it's a unit type
-        unsafe { &mut SQLITE_METADATA_LOOKUP }
+        &mut self.metadata_lookup
     }
 }
 
@@ -539,6 +539,7 @@ impl SqliteConnection {
             statement_cache: StatementCache::new(),
             raw_connection,
             transaction_state: AnsiTransactionManager::default(),
+            metadata_lookup: (),
             instrumentation: None,
         };
         conn.register_diesel_sql_functions()