[psql] Add support for describing auxiliary tables for ao table. (#14063)
[psql] Add support for describing auxiliary tables for ao table. (#14063)

This patch adds support for describing auxiliary tables. e.g.,

```
postgres=# create table foo(i int) with (appendonly=true);

postgres=# create index on foo(i);
CREATE INDEX

postgres=# select blkdirrelid::regclass from pg_appendonly where relid = 'foo'::regclass;
        blkdirrelid
----------------------------
 pg_aoseg.pg_aoblkdir_16411
(1 row)

postgres=# \d+ pg_aoseg.pg_aoblkdir_16411
Appendonly block directory table: "pg_aoseg.pg_aoblkdir_16411"
     Column     |  Type   | Storage
----------------+---------+---------
 segno          | integer | plain
 columngroup_no | integer | plain
 first_row_no   | bigint  | plain
 minipage       | bytea   | plain
Indexes:
    "pg_aoblkdir_16411_index" PRIMARY KEY, btree (segno, columngroup_no, first_row_no)

postgres=# \d+ pg_aoseg.pg_aovisimap_16411
Appendonly visibility map table: "pg_aoseg.pg_aovisimap_16411"
    Column    |  Type   | Storage
--------------+---------+---------
 segno        | integer | plain
 first_row_no | bigint  | plain
 visimap      | bytea   | plain
Indexes:
    "pg_aovisimap_16411_index" PRIMARY KEY, btree (segno, first_row_no)

postgres=# \d+ pg_aoseg.pg_aoseg_16411
Appendonly segment entry table: "pg_aoseg.pg_aoseg_16411"
     Column      |   Type   | Storage
-----------------+----------+---------
 segno           | integer  | plain
 eof             | bigint   | plain
 tupcount        | bigint   | plain
 varblockcount   | bigint   | plain
 eofuncompressed | bigint   | plain
 modcount        | bigint   | plain
 formatversion   | smallint | plain
 state           | smallint | plain
```
greenplum-db/gpdb,lisakowen/gpdb,lisakowen/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,xinzweb/gpdb,greenplum-db/gpdb,lisakowen/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,xinzweb/gpdb,lisakowen/gpdb,xinzweb/gpdb,lisakowen/gpdb,50wu/gpdb,xinzweb/gpdb,50wu/gpdb