Is your feature request related to a problem? Please describe.
There is currently no dedicated type provided for the mechanism, which leads to confusion regarding case sensitivity. For example, when using the SHA-SCRAM-512 mechanism, specifying it in uppercase resulted in an authorization error.

Describe the solution you'd like
I propose to introduce a dedicated type (e.g., SASLMechanism) for the mechanism field in KafkaJS. This change would enforce valid mechanism values and prevent case sensitivity issues. Alternatively, updating the documentation to specify that the mechanism must be provided in lowercase could also resolve the issue. Links to similar solutions in other Kafka clients or relevant KIPs would be valuable references.

Additional context
I encountered an authorization error when the mechanism was specified in uppercase, but switching to lowercase resolved the issue. This enhancement would help prevent similar errors in the future and improve overall type safety.
use type