PDO: Support IAM Authentication Tokens via Clear Text

[Videotapes]

Description

In order to make an RDS connection with an AWS IAM authentication token, it must be sent in clear/plain text. PDO does not currently support this (though it seems someone took a stab at it here: #18252 and the PR has been abandoned).

I believe the lack of clear text support may also be problematic for other token-based authentication methods.