Add public shutdown/close API

[marandaneto]

Summary

Add a public idempotent shutdown/close API.

Why

sdk-specs marks shutdown as applicable to both client and server SDKs. Backend SDKs should provide an explicit end-of-life hook that drains queued events and stops workers/timers.

Current status

The SDK has a supervision tree, but no public SDK-level shutdown/close API. Python has a related issue about bounded flush/shutdown behavior: PostHog/posthog-python#79.

Suggested scope

• Add public shutdown/close API for the default and named instances.
• Stop accepting new events after shutdown or make post-shutdown behavior explicit.
• Attempt a bounded final flush.
• Stop sender workers/timers and any future pollers/cache providers.
• Make repeated calls safe/idempotent.
• Add tests and docs.

References

• https://github.com/PostHog/sdk-specs/blob/main/openspec/specs/shutdown/spec.md