aws_dynamodb_cdc: support DynamoDB Global Tables for the checkpoint t…

[squiidz]

aws_dynamodb_cdc: support DynamoDB Global Tables for the checkpoint table

Add `global_table` / `global_table_replicas` options so the auto-created
checkpoint table is provisioned as a DynamoDB Global Table (v2), replicating
shard checkpoints across regions for low-RPO multi-region failover.

Checkpoints are keyed by a region-portable identifier (the source table name)
and store the stream ARN and record timestamp. Resume is layered: same-region
restarts resume exactly via AfterSequenceNumber (unchanged behavior), while a
failed-over region resumes from the trim horizon, skipping records at or before
the low-water-mark timestamp of the prior region's checkpoints. Table creation
enables DynamoDB Streams and provisions/reconciles replica regions; all new
behavior is gated behind `global_table` and is a no-op when disabled.

[claude]

Commits
LGTM

Review
Reviewed the addition of DynamoDB Global Table support for the aws_dynamodb_cdc checkpoint table (portable table-name key in global mode, replica reconciliation, time-based failover resume, and the new dynamodb_approximate_creation_time metadata / dynamodb_cdc_failover_skipped metric).

The change is well-structured and follows project patterns: the CheckpointerConfig struct replaces positional args, errors are wrapped with %w in gerund form, context is propagated correctly, field names use constants, the resume cache is synchronized via sync.Once, license headers match the package (RCL/enterprise), and the failover skip logic (\!After(cutoff)) matches the documented at-least-once design with proper nil guards. The parallel lastTimestamps map is bounded alongside lastCheckpoints, and both single- and multi-table read paths are wired consistently. New unit tests cover checkpointer create/reconcile/resume, batcher timestamp tracking, and config parsing/validation.

LGTM