[feat][io] Add Debezium Spanner source connector

[david-streamlio]

Motivation

Debezium ships a Google Cloud Spanner source connector (io.debezium:debezium-connector-spanner, present in the debezium-bom at our pinned version), but this repository does not wrap it. We currently wrap 5 of Debezium's 12 source connectors: MySQL, PostgreSQL, MongoDB, Oracle, and SQL Server.

Spanner is a managed, horizontally-scaled database with a growing Pulsar user base, and the connector itself wraps cleanly — the interesting work is in testing.

Proposal

Add a debezium/spanner module (pulsar-io-debezium-spanner) following the structure of the existing Debezium modules:

• DebeziumSpannerSource extending DebeziumSource
• include("debezium:pulsar-io-debezium-spanner") in settings.gradle.kts
• debezium-connector-spanner = { module = "io.debezium:debezium-connector-spanner" } in the version catalog
• NAR packaging + distribution wiring
• An integration test against the Spanner emulator (gcr.io/cloud-spanner-emulator/emulator) so CI needs no GCP credentials

Notes

• Spanner CDC works via change streams, which must be created with DDL (CREATE CHANGE STREAM ...) as part of test setup. Confirm the emulator supports change streams at the version we target — if it does not, the test must be credential-gated, and we should avoid repeating the azure-data-explorer situation ([improve][test] Azure Data Explorer E2E test is always skipped in CI — add a runnable integration test #49) where a credential-gated E2E test means the connector is never actually exercised in CI.
• Spanner is a multi-partition connector, so it will likely hit the task.id gap described in [Bug] Debezium SQL Server source fails to start: multi-partition connector never receives task.id #61. That issue should land first.
• Use the GenericContainer pattern (see the Testcontainers version conflict noted in [improve][test] Add Debezium Oracle source integration test #62).

Identified by a scan of the Debezium connector catalog against our connector inventory.