Add posibility to define connection hooks

[lchelmickiolx]

• [ X] Do only one thing
• [ X] Non breaking API changes
• [ ...] Tested

What did this pull request do?

It gives the possibility to define BeforeConnect and AfterConnect on stdlib connection.

User Case Description

Generate an IAM Auth password for RDS just before a new connection is opened. Tokens are valid for only 15 minutes, so after w while they have to be regenerated, for the pool to be able to open new connections

[jinzhu]

Hi @lchelmickiolx

How about passing the connection pool like this:

conn := stdlib.OpenDB(*config, OptionOpenDB...)

postgres.New(postgres.Config{
  Conn: conn,
})

The change seems to bind the current driver into the GORM driver too much, we doesn't want to do that, thank you for your unerstanding.