Add server-side alias helper

Summary

Add a public server-side alias helper.

Why

sdk-specs marks alias as applicable to both client and server SDKs. Backend SDK users should be able to link a previous/existing ID to a new ID without manually constructing $create_alias events.

Current status

No public alias API is present in public_api.snapshot.

Suggested scope

• Add an idiomatic Elixir API for $create_alias.
• Accept previous/existing distinct ID and new alias ID.
• Validate both IDs are present/non-empty.
• Emit event: "$create_alias", top-level distinct_id as the previous ID, and properties.alias as the new ID.
• 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/alias/spec.md

Start with public_api.snapshot and the sdk-specs alias specification, then trace the existing capture and batching entry points. Add the public server-side alias API with validation, normal capture/batching and before_send routing, and add tests and documentation that verify the specified event and IDs.