add the option to iamAuthentication in aws

[romga]

Aws has the ability to set iam authentication with RDS.

We have added this ability to the operator

[hitman99]

This is cool, I hope I can check this PR soon.

[eightnoneone]

@romga Thanks for doing this. It's a requirement for how we plan to authenticate Postgres RDS connections going forward.

I'm no Go developer, but looks like the code that handles Azure fails it's checks since you've added the additional param to the CreateUserRole(). Can a function's params be configured as optional in Go?

It wasn't clear from the code, but does this deal with the operator then not creating and storing a password in the Secret? I imagine one could alter the secretTemplate to not include it for each spec.