feat(kafka): support Avro-encoded message key via schema registry

[burnerlee]

PR checklist:

• Did you run ClangFormat? — new code follows surrounding style; all edited files are in Proton-specific paths
• Did you separate headers to a different section in existing community code base? — new includes added in alphabetical order within existing include blocks
• Did you surround proton: starts/ends for new code in existing community code base? — not applicable, all changes are in src/Storages/ExternalStream/Kafka/ which is Proton-specific code

Please write user-readable short description of the changes:

Adds a new message_key_schema_name setting to Kafka external streams. When set, Proton decodes the Kafka message key using the Confluent Avro wire format (5-byte wire prefix: magic byte + 4-byte schema ID) and emits the decoded value as a JSON string into _tp_message_key.

Motivation: Producers that use Confluent Schema Registry commonly Avro-encode message keys, not just values. Reading _tp_message_key on such topics previously returned raw bytes.

Behaviour:

• message_key_schema_name names an Avro schema subject in the schema registry
• Requires kafka_schema_registry_url (validated at CREATE EXTERNAL STREAM time)
• _tp_message_key column must be String or Nullable(String) in Avro key mode (validated at CREATE time)
• Schema lookup is cached via the existing KafkaSchemaRegistryForAvro mechanism
• On read: Avro binary key is decoded into a GenericDatum, then re-encoded as Avro JSON (not plain JSON — union types use Avro union encoding, bytes fields are base64). The result is stored as a string in _tp_message_key
• On write: throws NOT_IMPLEMENTED if message_key_schema_name is set — Avro-encoding keys on write is not yet supported. Plain string keys via _tp_message_key without message_key_schema_name continue to work
• Empty key (key_len == 0) returns NULL for Nullable(String) columns, empty string for String columns

Example:

CREATE EXTERNAL STREAM orders (
    _tp_message_key String,
    payload         String
) SETTINGS
    type                       = 'kafka',
    brokers                    = 'localhost:9092',
    topic                      = 'orders',
    data_format                = 'JSONEachRow',
    kafka_schema_registry_url  = 'http://localhost:8081',
    message_key_schema_name    = 'orders-key';

SELECT _tp_message_key FROM orders;
-- Returns Avro JSON string, e.g. {"user_id": 42, "tenant": "acme"}

Closes #915

[yuzifeng1984]

Need implement KafkaSink related things to encode message key or throw an exception on write if currently not supported.

[burnerlee]

@yuzifeng1984 how can I test my changes in local - can you share your testing setup?