Support mysql variable select stmt

[jd-zhang]

*Issue migrated from trac ticket # 885 www.kunlunbase.com *

component: computing nodes | priority: major

2022-07-08 11:16:45: zhaowei@zettadb.com created the issue

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we have already supported syntax to access variables in kunlun-storage, as described here: http://www.kunlunbase.com:8181/docs/b/b-1dnbk6om8uuha

The only thing left unsupported yet is this grammar:
SELECT @@session.var1 [[, @@global.var2],...];

This is needed for rust mysql client library to connect to Kunlun-server nodes, so we need to support it.

We do not support fetching such variables in the generic SELECT syntax as mysql, e.g. we don't support this syntax:

select @@session.autocommit, @@global.read_only, a from t1;

[jd-zhang]

2022-07-08 21:15:01: zhaowei@zettadb.com changed owner from david to kunlun

[jd-zhang]

2022-07-08 21:15:01: zhaowei@zettadb.com changed status from assigned to accepted