Support global and session parts in show variables for mysql and generic dialects

[emin100]

Examples:
mysql_and_generic().verified_stmt("SHOW GLOBAL VARIABLES");
mysql_and_generic().verified_stmt("SHOW GLOBAL VARIABLES LIKE 'admin%'");
mysql_and_generic().verified_stmt("SHOW GLOBAL VARIABLES WHERE value = '3306'");
mysql_and_generic().verified_stmt("SHOW SESSION VARIABLES");
mysql_and_generic().verified_stmt("SHOW SESSION VARIABLES LIKE 'admin%'");
mysql_and_generic().verified_stmt("SHOW GLOBAL VARIABLES WHERE value = '3306'");

[tobyhede]

👍 Seems like a simple change with solid set of tests

[coveralls]

Pull Request Test Coverage Report for Build 6926290207

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• 12 of 14 (85.71%) changed or added relevant lines in 3 files are covered.
• 517 unchanged lines in 9 files lost coverage.
• Overall coverage increased (+0.06%) to 87.707%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/ast/mod.rs	9	11	81.82%

Files with Coverage Reduction	New Missed Lines	%
tests/sqlparser_mysql.rs	2	99.42%
src/ast/data_type.rs	3	81.93%
src/lib.rs	5	11.83%
tests/sqlparser_mssql.rs	9	97.36%
tests/sqlparser_postgres.rs	33	98.06%
src/ast/query.rs	61	83.81%
src/tokenizer.rs	80	88.62%
tests/sqlparser_common.rs	120	96.91%
src/parser/mod.rs	204	83.16%

Totals
Change from base Build 6683069102:	0.06%
Covered Lines:	17801
Relevant Lines:	20296

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💛 - Coveralls

[alamb]

Thank you @emin100 and @tobyhede (I really appreciate the help reviewing!)