Easier to fetch tag/edge_type comment

[wey-gu]

We cannot get tag/edge_type level comment now, it can be only shown in show create tag as follows, we may introduce new Semitic or add extra information in existing queries for this information in a field to make the application easier to get comments of tag/edge_type.

(root@nebula) [basketballplayer]> create edge hello(name string comment "name of hello") comment="helloworld for edge type";
Execution succeeded (time spent 6957/56438 us)

Tue, 26 Jul 2022 11:57:59 CST

(root@nebula) [basketballplayer]> desc edge hello
+--------+----------+-------+---------+-----------------+
| Field  | Type     | Null  | Default | Comment         |
+--------+----------+-------+---------+-----------------+
| "name" | "string" | "YES" |         | "name of hello" |
+--------+----------+-------+---------+-----------------+
Got 1 rows (time spent 1160/53604 us)

Tue, 26 Jul 2022 11:58:04 CST

(root@nebula) [basketballplayer]> show create edge hello
+---------+-------------------------------------------------------------------------+
| Edge    | Create Edge                                                             |
+---------+-------------------------------------------------------------------------+
| "hello" | "CREATE EDGE `hello` (                                                  |
|         |  `name` string NULL COMMENT "name of hello"                             |
|         | ) ttl_duration = 0, ttl_col = "", comment = "helloworld for edge type"" |
+---------+-------------------------------------------------------------------------+
Got 1 rows (time spent 1089/72901 us)

Tue, 26 Jul 2022 11:58:10 CST

(root@nebula) [basketballplayer]> show edges
+----------+
| Name     |
+----------+
| "follow" |
| "hello"  |
| "serve"  |
+----------+