Add simple query

[alswang18]

Added support for simple query protocol used by postgres-rust package.

Issues: https://github.com/sfu-db/connector-x/issues/281

[alswang18]

Thanks Alec, the code looks great! I left a few comments.

A reminder is that you can use cargo fmt to format the code before submission. Otherwise our CI will not pass

got it thanks for the reminder xiaoying :) just ran it