Restrict the use of client.security.sasl.jaas.config to PlainLoginModule exclusively.

[loserwang1024]

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Description

Currently, we support client.security.sasl.username and client.security.sasl.password. Unlike Kafka, our server does not support custom SASL mechanisms—instead, we provide a more flexible, plugin-based authentication system. As a result, client.security.sasl.jaas.config is no longer necessary.

However, for backward compatibility, client.security.sasl.jaas.config must still be retained. Since only SASL/PLAIN is currently supported, we can minimize the scope of this configuration to PlainLoginModule only.

Willingness to contribute

• I'm willing to submit a PR!

[loserwang1024]

@wuchong , WDYT?