Support RDS IAM Auth

[mightyguava]

Feature Request

Is your feature request related to a problem? Please describe:
We use Aurora for most of our databases, and would strongly prefer avoiding using passwords for ad-hoc tasks like database dumps. Aurora supports RDS IAM Authentication. When we run dumpling, we usually create a new auth token, valid for 15 minutes, to use for auth. Since dumpling initializes its pool of connections on startup and keeps them around forever, this works.

However, if a connection is interrupted and dumpling retries #178, the auth token may no longer be valid.

Describe the feature you'd like:
Dumpling can be configured to generate an RDS auth token itself using provided IAM credentials, instead of needing one provided at startup. This way dumpling can regenerate the token on the retry.