[FEATURE REQUEST] support AWS MSK IAM authentication

Why this feature?

The current Kafka configuration supports username/password authentication, and #141 adds configurable PLAIN and SCRAM mechanisms. AWS MSK IAM authentication is a separate mechanism that cannot be enabled by setting a username and password.

AWS MSK IAM requires SASL/OAUTHBEARER, a dynamic SigV4 token provider, TLS, an AWS region, and credentials resolved through the AWS default credential chain.

Suggested Solution

Add a backward-compatible AWS_MSK_IAM authentication mechanism that:

• maps to Sarama OAUTHBEARER
• generates tokens with the official github.com/aws/aws-msk-iam-sasl-signer-go package
• requires an AWS region
• uses the AWS default credential chain without adding static access keys to Kafka configuration
• enables TLS automatically
• is shared by producer, consumer, topic verification, and health checks
• supports unit testing through an injectable token generator without requiring a real AWS account

This should use a separate issue and branch from #141 according to the OpenIM one-issue-one-branch contribution workflow. The implementation can build on #141 and be rebased onto main after #141 is merged.

Additional Information

AWS reference: https://github.com/aws/aws-msk-iam-sasl-signer-go

Begin by reviewing the implementation from #141 and the AWS MSK IAM SASL signer reference package, then trace where Kafka configuration is shared by producers, consumers, topic verification, and health checks. Done means adding the backward-compatible AWS_MSK_IAM mechanism with region and default-credential handling, automatic TLS, and injectable token generation suitable for unit tests without a real AWS account.