feat: Support SET DEFAULT syntax (#251)

Signed-off-by: Yihai Lin <yihai-lin@foxmail.com>

proto/greptime/v1/ddl.proto

@@ -119,6 +119,7 @@ message AlterTableExpr {
     DropDefaults drop_defaults = 14;
     SetIndexes set_indexes = 15;
     UnsetIndexes unset_indexes = 16;
+    SetDefaults set_defaults = 17;
   }
 }
 
@@ -134,6 +135,12 @@ message UnsetIndexes {
   repeated UnsetIndex unset_indexes = 1;
 }
 
+message SetDefault {
+  string column_name = 1;
+  // The JSON representation of the default `Expr`.
+  bytes default_constraint = 2;
+}
+
 message SetIndex {
   oneof options {
     SetFulltext fulltext = 1;
@@ -186,6 +193,10 @@ message DropDefaults {
   repeated DropDefault drop_defaults = 1;
 }
 
+message SetDefaults {
+  repeated SetDefault set_defaults = 1;
+}
+
 message DropColumns {
   repeated DropColumn drop_columns = 1;
 }