Add server-side group_identify helper #143

Summary

Add a public server-side group identify helper.

Why

sdk-specs marks group-identify as applicable to both client and server SDKs. Server SDKs commonly expose this directly to create/update group profiles.

Current status

No public group identify API is present in public_api.snapshot.

Suggested scope

• Add an idiomatic Elixir API for $groupidentify.
• Accept group type, group key, and optional group properties.
• Emit $group_type, $group_key, and $group_set/properties according to backend SDK behavior.
• Support an optional distinct ID or synthesize one from group type/key if omitted.
• Route through normal capture/batching and future before_send hook.
• Add tests and docs.

References

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