feat: add HTTP and NATS output sinks

[mostafa]

Summary

• Add HttpSink -- POST batched NDJSON lines to a remote HTTP endpoint using reqwest. Lines are batched by count (batch_size, default 100) or time (batch_timeout_ms, default 500ms) and sent as a single NDJSON body. Retries on 5xx/timeout with exponential backoff (max_retries, default 3). Custom headers configurable in YAML (e.g. Authorization).
• Add NatsSink -- publish each NDJSON line to a NATS subject via core publish (fire-and-forget) using async-nats. Behind the nats feature flag to keep the dependency optional. Subject parsed from the URL path (nats://host:port/subject) or overridden via global.output.nats.subject.
• Both sinks are selectable via URL-scheme detection on the existing --output CLI flag: --output http://... for HTTP, --output nats://... for NATS, plain paths remain file output, absent remains stdout.
• Both implement the existing sync EventSink trait using a tokio channel + spawned async worker, and slot into the existing BackpressureSink wrapper without changes.
• New config section global.output with http and nats sub-blocks for optional tuning.
• Updated README with new output types, quick-start examples, and config reference entries.

Files changed

File	What
Cargo.toml	async-nats optional dep, nats feature flag
src/config/global.rs	OutputConfig, HttpOutputConfig, NatsOutputConfig
src/output.rs	HttpSink, NatsSink, parse_nats_url(), tests
src/main.rs	Sink construction with URL-scheme detection
README.md	Documentation for new output sinks

Test plan

• Unit tests for parse_nats_url (4 cases: with subject, nested subject, no subject, trailing slash)
• Integration test for HttpSink basic POST (wiremock mock server, verifies batch delivery)
• Integration test for HttpSink retry on 503 (wiremock, verifies 3 attempts before success)
• cargo clippy clean (default features and --features nats)
• cargo test passes (all 139 tests)