Add identify and person property helpers

[marandaneto]

Summary

Add first-class user identification and person property helper APIs to the Rust SDK, matching the ergonomics of other backend SDKs.

Motivation

The Rust SDK can already capture identified events by passing a distinct_id to Event::new, and users can manually set person properties by adding $set, $set_once, or $unset properties to an event.

However, compared with other backend SDKs, this is less discoverable and makes the SDK look like it does not support user identification well. For example:

• PHP exposes PostHog::identify(...) for setting person properties.
• Node exposes setPersonProperties(...), unsetPersonProperties(...), and helper capture APIs.
• Python documents identify_context(...) and uses $set / $set_once in capture(...).
• Ruby exposes identify(...).

Rust users currently need to know the raw PostHog event/property protocol and construct those payloads manually.

Proposed API

Exact naming can be adjusted to fit Rust conventions, but something like:

client.identify(
    "user_distinct_id",
    IdentifyOptions::new()
        .set("email", "max@example.com")?
        .set("name", "Max Hedgehog")?
        .set_once("initial_url", "/blog")?,
).await?;

And/or explicit person property helpers:

client.set_person_properties(
    "user_distinct_id",
    PersonProperties::new()
        .set("name", "Max Hedgehog")?
        .set_once("initial_url", "/blog")?,
).await?;

client.unset_person_properties("user_distinct_id", ["email"]).await?;

Blocking client equivalents should be available too:

client.identify("user_distinct_id", options)?;
client.set_person_properties("user_distinct_id", properties)?;
client.unset_person_properties("user_distinct_id", ["email"])?;

Under the hood, these can send the same $set event / $set, $set_once, and $unset payloads that users can send manually today.

Acceptance criteria

• Async and blocking clients expose helpers for identifying a user and setting person properties.
• Helpers support $set and $set_once values.
• A helper exists for unsetting person properties ($unset) or there is a documented reason to leave it out.
• Helpers accept serializable values and surface serialization errors clearly.
• Tests cover the generated event payloads.
• Rust docs can show the helper APIs instead of requiring manual $set, $set_once, and $unset event construction.

Related issues

• Add alias helper #149 adds an alias helper.
• Add group_identify helper #150 adds a group_identify helper.
• Add request-scoped context and web middleware helpers #151 covers request-scoped context and middleware helpers.