pulsar: support debezium protocol

Is your feature request related to a problem?

The Pulsar sink only accepts canal-json as the message protocol. Users who consume TiCDC change events via Pulsar with Debezium-compatible downstream systems (e.g. Flink CDC, Debezium connectors) have no way to produce standard Debezium-format messages, even though the Debezium codec is already implemented and used by the Kafka sink.

Describe the feature you'd like

Allow protocol=debezium to be set in the Pulsar sink URI:

pulsar://host:6650/topic?protocol=debezium

This requires extending IsPulsarSupportedProtocols() in pkg/config/sink_protocol.go to include ProtocolDebezium. No new codec logic is needed since the Debezium encoder/decoder in pkg/sink/codec/debezium/ is already shared with the Kafka sink via the common codec builder.

Describe alternatives you've considered

Users currently must use canal-json and implement a custom transform layer in their consumer to convert to Debezium format. This adds operational overhead and is error-prone.

Teachability, Documentation, Adoption, Migration Strategy

The Pulsar sink URI parameter documentation should be updated to list debezium as a valid value for the protocol parameter alongside canal-json. No migration is required for existing changefeeds since the default protocol is unchanged.

Start in pkg/config/sink_protocol.go at IsPulsarSupportedProtocols(), then inspect the shared codec handling under pkg/sink/codec/debezium/. Update the Pulsar sink protocol options so protocol=debezium is accepted, and update the Pulsar sink URI documentation to list it alongside canal-json; existing defaults should remain unchanged.