db2: add db2_cdc input connector

Full-featured IBM DB2 CDC connector built on SQL Replication (ASNCDC).
Supports initial full-table snapshot, live CDC streaming, and incremental
on-demand snapshots via a signal table (compatible with Debezium's
execute-snapshot protocol). Key capabilities:

- Snapshot modes: initial, always, never, no_data
- Configurable snapshot isolation (DB2 RR or CS), batch size, parallelism
- Per-chunk checkpoint persistence for crash-safe incremental snapshot resume
- Signal table: execute-snapshot, stop-snapshot, pause/resume-snapshot
- Table regex include/exclude filters
- Debezium-envelope output (op c/u/d/r, before/after, source metadata)
- Schema-change events with column-metadata cache invalidation
- Heartbeat events for consumer lag detection
- checkpoint_cache support (Redpanda Connect cache resource or DB2 table)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: emaxerrno