Add FedifySpanExporter for distributed trace storage

Implement FedifySpanExporter class that persists ActivityPub activity
traces to a KvStore, enabling distributed tracing across multiple nodes.
This allows building debug dashboards that show complete request flows
across web servers and background workers.

Features:

- Captures inbound (received) and outbound (sent) activities from
  OpenTelemetry span events
- Supports both list() and cas() storage strategies depending on
  KvStore capabilities
- Includes actorId and signatureDetails fields for debug dashboard use
- Provides getActivitiesByTraceId() and getRecentTraces() query methods
- Configurable TTL and key prefix options

Closes https://github.com/fedify-dev/fedify/issues/497



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