Kafka Connect: Make KafkaMetadataTransform configuration per-instance

[wombatu-kun]

Closes #16601

Problem

KafkaMetadataTransform held its configuration (recordAppender) in a static field, so every instance in a worker JVM shared one configuration and the last configure() call won. Two differently-configured instances (two connectors that use the SMT, or two transform aliases in one chain) would emit each other's metadata field names and structure. It is also a thread-safety hazard, since the field is mutated from each task's configure() and read from each task's apply(). The other SMTs in the module (CopyValue, DebeziumTransform) already use instance fields.

Solution

Make recordAppender an instance field. All reads are in instance methods, so the change is minimal, and the appender is still built once per instance in configure(), so there is no added cost.

Tests

Added TestKafkaMetadataTransform.testConfigIsNotSharedAcrossInstances: it configures two instances with different field names and verifies each instance uses its own configuration. It fails before the change and passes after.