feat(query): Support SHOW VARIABLES

[TCeason]

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Summary

Support SHOW VARIABLES

add table function show_variables()

Note:

MySQL also support show variables , so under MySQL handler please use select * from show_variables() to query all vars in current session.

• fixes: show variables #16274

Tests

• Unit Test
• Logic Test
• Benchmark Test
• No Test - Explain why

Type of change

• Bug Fix (non-breaking change which fixes an issue)
• New Feature (non-breaking change which adds functionality)
• Breaking Change (fix or feature that could cause existing functionality not to work as expected)
• Documentation Update
• Refactoring
• Performance Improvement
• Other (please describe):

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This change is 

[TCeason]

cc @BohuTANG Please review this pr. And Do not need to modify doc.