feat: add flow_id and drop_if_exists to DropFlowExpr (#164)

* feat: add `flow_id` and `drop_if_exists` to `DropFlowExpr`

* chore: apply suggestions from CR

* chore: generate code

proto/greptime/v1/ddl.proto

@@ -56,6 +56,8 @@ message CreateFlowExpr {
 message DropFlowExpr {
   string catalog_name = 1;
   string flow_name = 2;
+  FlowId flow_id = 3;
+  bool drop_if_exists = 5;
 }
 
 message CreateTableExpr {
@@ -135,6 +137,8 @@ message DropColumn { string name = 1; }
 
 message TableId { uint32 id = 1; }
 
+message FlowId { uint32 id = 1; }
+
 message ColumnDef {
   string name = 1;
   ColumnDataType data_type = 2;