Support STS AssumeRole for cross-account IAM authentication (PostgreSQL / MySQL)

Hello

## Feature request or enhancement

_For feature requests or enhancements, please provide the following information:_

**Which use case/requirement will be addressed by the proposed feature?**

I'm running Kafka Connect (MSK Connect) in AWS account A and need to capture changes from RDS PostgreSQL databases in AWS account B using IAM authentication (database.authmode = aws-iam).

The connector generates the RDS IAM auth token using the credentials available to the process (the MSK Connect execution role in account A). RDS in account B rejects this token because the principal isn't in the same account.

The standard AWS pattern here is to assume a role in account B via STS AssumeRole, then generate the token with those temporary credentials. The AWS Advanced JDBC Wrapper supports this through StsAssumeRoleCredentialsProvider (see https://github.com/aws/aws-advanced-jdbc-wrapper/discussions/1239), but it requires programmatic configuration. I couldn't find a way to pass a role ARN through Debezium connector properties.

A property like database.iam.role.arn that tells the connection factory to assume a given role before generating the IAM auth token would make cross-account CDC possible without falling back to static credentials.

I might have missed an existing way to do this though, happy to be corrected

Have a good day !

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**Implementation ideas (optional)**

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