Support AWS MSK Authentication

### Is your feature request related to a problem? Please describe.

Currently using fluent with AWS MSK as unauthenticated with security groups to control access. Want to enable AWS IAM assume role for cross account access.

### Describe the solution you'd like

Same setup as fluent-plugin-s3 for **assume_role_credentials** would be good.
e.g https://github.com/fluent/fluent-plugin-s3/blob/master/docs/credentials.md

### Describe alternatives you've considered

Only alternative is to continue as unauthenticated.

### Additional context

_No response_