💡 Motivation and Context

Closes #150.

Adds a first-class group_identify helper so users no longer need to hand-build the $groupidentify event:

client.group_identify(
    "company",
    "company_id_in_your_db",
    json!({
        "name": "Awesome Inc.",
        "employees": 11,
    }),
);

Available on both the async and blocking clients with the same signature.

Implementation Details & Design Decisions

1. Payload Structure & Wire Format

   • Emits event name "$groupidentify".
   • The top-level distinct_id is set to format!("${group_type}_{group_key}") (e.g. "$company_company_id_in_your_db"), matching posthog-js (posthog-core-stateless / posthog-node), posthog-python, and the PostHog group analytics specification.
   • Properties include $group_type, $group_key, and $group_set: properties.
   • Person profile processing is preserved by default (group identification is personful).

2. Synchronous Construction & Fire-and-Forget Delivery

   • Matching capture and alias, the helper synchronously constructs and enqueues the event on both async and blocking clients; delivery remains fire-and-forget.
   • Accepts generic group_type: impl Into<String>, group_key: impl Into<String>, and properties: impl Serialize.
   • Blank group_type or group_key cannot describe a valid group identification and are dropped with a warning, matching posthog-python.
   • Property serialization failures and values that do not serialize to a JSON object return Error::Serialization to the caller.
   • Event construction lives in Event::group_identify so both clients share the exact same payload definition and validation logic.

💚 How did you test it?

• Added tests/test_group_identify.rs covering both async_client and blocking clients across all key scenarios:
  • Well-formed $groupidentify payload verification ($group_type, $group_key, $group_set)
  • Person profile processing is preserved
  • Generic string and struct serialization ergonomics
  • Distinct event UUID generation across repeated calls
  • Blank group_type and group_key drops
  • Disabled client no-op
• Added unit tests in src/event.rs:
  • v0_group_identify_payload
  • group_identify_rejects_blank_keys
• Verified:
  • cargo test --test test_group_identify (6 passed)
  • cargo test --no-default-features --test test_group_identify (6 passed)
  • cargo test --lib event (34 passed)
  • cargo test --doc (7 passed)
  • cargo clippy -- -D warnings (clean)
  • cargo fmt --check (clean)
  • api/public-api.txt snapshot updated
  • Added changeset in .sampo/changesets/add-group-identify-helper.md

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change.
• Included changeset file (.sampo/changesets/add-group-identify-helper.md).