实例配置增加默认字符集

[hhyo]

关联issue：#158 #214 #249

• 实例配置中新增字符集字段，可为空
• 为空时mysql默认使用utf8mb4，mssql默认使用UTF8

[LeoQuote]

oracle 和pg的要怎么设置？

[hhyo]

不了解，所以是保持现状

[codecov]

Codecov Report

Merging #274 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #274      +/-   ##
==========================================
+ Coverage   83.75%   83.75%   +<.01%     
==========================================
  Files          65       65              
  Lines        8410     8411       +1     
==========================================
+ Hits         7044     7045       +1     
  Misses       1366     1366

Impacted Files	Coverage Δ
sql/completer/mysql.py	100% <ø> (ø)	⬆️
sql/engines/mysql.py	91.16% <ø> (ø)	⬆️
sql/models.py	94.92% <100%> (ø)	⬆️
sql/engines/goinception.py	91.3% <100%> (ø)	⬆️
sql/engines/mssql.py	84.17% <100%> (ø)	⬆️
sql/engines/inception.py	88.62% <100%> (ø)	⬆️

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