add query hint support

[dozer47528]

What did this pull request do?

We are using our own MySQL read/write proxy.
In order to implement select on master db, we will add some sql hint to our sql query.

For example:
select * from users where id = 1

Force use master db:
/*master*/ select * from users where id = 1

It's a common use case for MySQL read/write and sharding.

[codecov-io]

Codecov Report

Merging #2351 into master will decrease coverage by 0.03%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #2351      +/-   ##
==========================================
- Coverage   79.42%   79.39%   -0.04%     
==========================================
  Files          24       24              
  Lines        3466     3470       +4     
==========================================
+ Hits         2753     2755       +2     
- Misses        612      613       +1     
- Partials      101      102       +1

Impacted Files	Coverage Δ
callback_row_query.go	84.61% <0%> (-15.39%)	⬇️
callback_query.go	82.75% <100%> (+0.61%)	⬆️

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