feat: support AWS IAM database authentication

[yashrith]

What does this change do?

This update adds support for AWS IAM Database Authentication (passwordless login) in tap-postgres.

Why is this helpful?

Currently, to connect to a Postgres database, you have to type in a permanent password in the configuration.

With this new feature, you can connect securely using temporary AWS IAM tokens instead. This means you don't have to save or share database passwords, which is much more secure and follows AWS security best practices.

How was it built?

1. Added new settings: You can now enable this feature using the aws_iam_auth toggle, and configure your AWS region and profile.
2. Automatic tokens: When the tap connects, it automatically asks AWS for a short-lived login token and uses it as the password.
3. Tests: Added a new test file to verify everything works correctly.