when using external schema, struct's fields are not shown in describe <table>

[xxchan]

when using external schema, struct's fields are not shown in describe <table>

https://github.com/risingwavelabs/risingwave/pull/17110/files#r1627299710

example:

query
describe kafka_json_schema_plain;
----
dimensions (empty) false NULL
map jsonb false NULL
notMap (empty) false NULL
price double precision false NULL
productId bigint false NULL
productName character varying false NULL
tags character varying[] false NULL
_row_id serial true NULL
primary key _row_id NULL NULL
distribution key _row_id NULL NULL
table description kafka_json_schema_plain NULL NULL

There should be dimenstions.xx and notMap.xx. BTW, maybe we can also show struct instead of null for them.

[xxchan]

It seems this is because ColumnDesc have too many constructors. And only some of them has type_name.

• At the beginning, we only have ColumnDesc::unnamed refactor(catalog): column desc #717
• Then we introduced new_atomic and new_struct feat: create nested table schema according to proto file #1078. We should have removed unnamed, and only use new_atomic since then, but we didn't.
• We forgot about the struct stuff, and added ColumnDesc::named.... feat(cdc): share a changelog stream for multiple cdc tables #12535 refactor(common): use ColumnDesc::named when possible #13574
• We can still create ColumnDesc directly.

[xxchan]

related #13104

[xxchan]

More specifically, ColumnDesc.field_descs is not set.