[plan] add show queries statement support

[sundy-li]

Summary

mysql> CREATE TABLE t(c1 int) ENGINE = Null;
Query OK, 0 rows affected (0.01 sec)

mysql> show tables;
+------+
| name |
+------+
| t    |
+------+
1 row in set (0.02 sec)

mysql> use system
Reading table information for completion of table and column names
You can turn off this feature to get a quicker startup with -A

mysql> show tables;
+------------+
| name       |
+------------+
| clusters   |
| one        |
| functions  |
| numbers    |
| settings   |
| numbers_mt |
| tables     |
+------------+
7 rows in set (0.01 sec)

mysql> show settings;
+----------------+
| name           |
+----------------+
| max_block_size |
| max_threads    |
| default_db     |
+----------------+
3 rows in set (0.01 sec)

Changelog

• New Feature

[codecov]

Codecov Report

Merging #156 (8f00a9d) into master (616a5df) will decrease coverage by 0%.
The diff coverage is 60%.

@@          Coverage Diff          @@
##           master   #156   +/-   ##
=====================================
- Coverage      82%    82%   -1%     
=====================================
  Files         180    180           
  Lines        7416   7436   +20     
=====================================
+ Hits         6087   6099   +12     
- Misses       1329   1337    +8     

Impacted Files	Coverage Δ
src/sql/plan_parser.rs	82% <0%> (-2%)	⬇️
src/sql/sql_parser.rs	73% <70%> (-1%)	⬇️
src/sql/sql_parser_test.rs	87% <100%> (+1%)	⬆️

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[BohuTANG]

LGTM