support gbk charset

[spongedu]

1. Support charset gbk in parser;
2. fix description for charsetlatin and charsetbin1 and charsetbin

What problem does this PR solve?

We're using parser to develop some our systems and found that charset gbk is not supported yet in parser, which makes some SQL fail to work.
As a MySQL compatible parser, I think we should support gbk as MySQL's parser do, and leave the charset-compatible issues to TiDB layer.

What is changed and how it works?

Add gbk into supported charsets, and add related tests

Check List

Tests

• Unit test
• Manual test

[codecov]

Codecov Report

Merging #808 into master will decrease coverage by 0.08%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #808      +/-   ##
==========================================
- Coverage   78.35%   78.27%   -0.09%     
==========================================
  Files          40       40              
  Lines       14773    14707      -66     
==========================================
- Hits        11576    11512      -64     
- Misses       2509     2515       +6     
+ Partials      688      680       -8     

[kennytm]

Rest LGTM.

Why do we support GBK rather than its superset GB18030?

[spongedu]

Rest LGTM.

Why do we support GBK rather than its superset GB18030?

@kennytm Because we just met gbk in our cases. maybe gb18030 should be supported as well

[tiancaiamao]

PTAL @bb7133 @wjhuang2016

[wjhuang2016]

GetSupportedCharsets need to change too.

[spongedu]

@wjhuang2016 PTAL :)

[ti-chi-bot]

@spongedu: PR needs rebase.

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