*: support show index syntax #679

[zhyass]

[summary]
Support show index syntax.

SHOW {INDEX | INDEXES | KEYS}
{FROM | IN} tbl_name
[{FROM | IN} db_name]
[WHERE expr]

[test case]
src/proxy/show_test.go
src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/show_test.go

[patch codecov]
src/proxy/query.go 88.7%
src/proxy/show.go 90.1%
src/vendor/github.com/xelabs/go-mysqlstack/sqlparser/ast.go 96.8%

[codecov]

Codecov Report

Merging #680 into master will decrease coverage by 0%.
The diff coverage is 93%.

@@          Coverage Diff           @@
##           master    #680   +/-   ##
======================================
- Coverage      90%     89%   -1%     
======================================
  Files         130     130           
  Lines       10501   10530   +29     
======================================
+ Hits         9453    9472   +19     
- Misses        704     711    +7     
- Partials      344     347    +3     

Impacted Files	Coverage Δ
src/proxy/show.go	83% <92%> (+<1%)	⬆️
src/proxy/query.go	86% <100%> (+<1%)	⬆️
src/backend/connection.go	88% <0%> (-5%)	⬇️
src/backend/pool.go	89% <0%> (-4%)	⬇️
src/executor/engine/merge_join.go	96% <0%> (ø)

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