KIP-506 — Allow setting SCRAM password via Admin interface

Discussion SecurityAdmin

Extends the `AdminClient` (and the underlying `AlterUserScramCredentials` RPC) to allow creating, altering, and deleting SCRAM credentials for Kafka users. SCRAM credential management currently requires running `kafka-configs.sh` with a direct ZooKeeper connection, making it impossible for application code to manage user credentials without a ZooKeeper dependency.

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