Add support for PniSignatureMessages.

core-util/src/main/java/org/signal/core/util/SQLiteDatabaseExtensions.kt

@@ -34,6 +34,15 @@ fun SupportSQLiteDatabase.getTableRowCount(table: String): Int {
   }
 }
 
+/**
+ * Checks if a row exists that matches the query.
+ */
+fun SupportSQLiteDatabase.exists(table: String, query: String, vararg args: Any): Boolean {
+  return this.query("SELECT EXISTS(SELECT 1 FROM $table WHERE $query)", SqlUtil.buildArgs(*args)).use { cursor ->
+    cursor.moveToFirst() && cursor.getInt(0) == 1
+  }
+}
+
 /**
  * Begins a SELECT statement with a helpful builder pattern.
  */