feat(pgsql): add support for concurrent index creation

* Implemented `Concurrent` field in index model
* Updated index creation template to support `CREATE INDEX CONCURRENTLY`
* Added tests for concurrent index creation in migration writer

pkg/writers/pgsql/writer.go

@@ -1097,8 +1097,13 @@ func (w *Writer) writeIndexes(schema *models.Schema) error {
 				whereClause = fmt.Sprintf(" WHERE %s", index.Where)
 			}
 
-			fmt.Fprintf(w.writer, "CREATE %sINDEX IF NOT EXISTS %s\n",
-				unique, indexName)
+			concurrently := ""
+			if index.Concurrent {
+				concurrently = "CONCURRENTLY "
+			}
+
+			fmt.Fprintf(w.writer, "CREATE %sINDEX %sIF NOT EXISTS %s\n",
+				unique, concurrently, indexName)
 			fmt.Fprintf(w.writer, "  ON %s USING %s (%s)%s;\n\n",
 				w.qualTable(schema.SQLName(), table.SQLName()), indexType, strings.Join(columnExprs, ", "), whereClause)
 		}