Feature: Add MSK IAM auth plugin with token refresh

[scottybrisbane]

The existing MSK IAM auth functionality built in to the proxy doesn't refresh the authentication token before expiry. As such, when the authentication expires, MSK force closes the connection between the proxy and the brokers. This forces the proxy to re-establish new connections with new authentication credentials, which causes unexpected EOF errors in proxy clients before the connection is re-established.

To improve the behaviour of the proxy when working with IAM authentication against MSK clusters, this pull request introduces a new auth token provider plugin that handles IAM based SASL authentication with MSK. The new plugin supports token refresh before expiry, thereby improving the behaviour described above. https://github.com/aws/aws-msk-iam-sasl-signer-go is used to handle the actual token generation.

[scottybrisbane]

Hmmm... on testing this further, it's still not quite solving the problem of connections between the proxy and brokers being closed by MSK. I suspect this is due to authentication on existing open connections not being refreshed, even when the tokens that were used for the initial auth have expired. I'll move this back to a draft state for now whilst I investigate further and consider other options.