Envelope and Encoding support

[vnvo]

feat: Configurable envelope formats and wire encoding for sinks
Adds support for multiple output envelope formats across all sinks, enabling Debezium compatibility and CloudEvents support without changing the internal event model.
Envelope Formats

Native - Direct Event serialization (default, lowest overhead)
Debezium - Wire-compatible with Debezium's JsonConverter schemaless mode ({"schema":null,"payload":{...}})
CloudEvents - CNCF CloudEvents 1.0 specification

Configuration
yamlsinks:

• type: kafka
  config:
  id: kafka-sink
  brokers: localhost:9092
  topic: events
  envelope:
  type: debezium # native | debezium | cloudevents
  encoding: json # json (default), avro planned
  Changes
  Core:

New envelope module with Native, Debezium, and CloudEvents implementations
New encoding module with Json encoder (extensible for Avro/Protobuf)
Event structure aligned with Debezium payload format internally

Sources:

MySQL and PostgreSQL sources refactored to adopt envelope/encode model
Fixed MySQL DDL handling and schema reload

Sinks:

All sinks (Kafka, Redis, NATS) updated to use configurable envelope and encoding
Per-sink envelope configuration via YAML

Tests:

All sink integration tests updated for envelope/encoding support
Turso integration tests temporarily disabled

Docs:

New envelopes.md documentation with wire format examples
Updated README with quick start and envelope configuration
Updated example configs

Migration
Existing pipelines continue to work unchanged (defaults to native envelope, json encoding). Users migrating from Debezium can set envelope: { type: debezium } for drop-in compatibility.

closes #36 and #37