Add server-side identify and person property update helpers

Summary

Add server-side public helpers for identifying users and updating person properties.

Why

sdk-specs marks identify as applicable to both client and server SDKs. Server SDKs should be able to emit stateless person profile updates without relying on callers to hand-build $identify, $set, or $set_once capture events.

Current status

public_api.snapshot exposes capture/3 and feature flag APIs, but no public identify, set, or set_once helpers.

Suggested scope

• Add an idiomatic Elixir API for server-side identify that emits $identify with $set and optional $set_once.
• Consider separate set / set_once helpers if that matches backend SDK parity better.
• Support standard event options where possible: timestamp, uuid, and disable_geoip.
• Route through the same capture/batching path and future before_send hook.
• Add tests and update public API docs/snapshot.

References

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

Start with the public_api.snapshot, existing capture/3 and feature flag APIs, then compare the sdk-specs identify specification. Define the public server-side identify behavior, including optional $set_once and standard event options, and add tests plus public API documentation or snapshot updates. Done means the helpers use the existing capture/batching path and their behavior is covered by tests.