KIP-1341 — Replace `<default>` placeholder in `ConfigEntity#name()` with `Optional<String> entityName()`

Discussion BrokerAdmin

Adds Optional<String> entityName() to ClientQuotaEntity.ConfigEntity, returning Optional.empty() for default quota entities, and deprecates name() for removal in the next major release. name() returns the literal string "<default>", a ZooKeeper znode-path artifact with no meaning in KRaft, which contradicts both its own Javadoc (which claims an empty string) and the rest of Kafka, where Admin client and KRaft records represent a default quota entity with null.

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