Add event deduplication via id and timestamp params

[hownowstephen]

Summary

• Change track() and track_anonymous() to accept a data dict instead of **kwargs
• Add optional id parameter (ULID) for event deduplication
• Add optional timestamp parameter (epoch seconds) for explicit event timing
• Invalid timestamps silently dropped; data=None sends empty data dict

This is a breaking change — callers must switch from keyword arguments to a dict for event data attributes:

# Before
cio.track(5, "purchase", type="socks", price="13.99")

# After
cio.track(5, "purchase", {"type": "socks", "price": "13.99"})
cio.track(5, "purchase", {"type": "socks"}, id="01HB4HBDKTFWYZCK01DMRSWRFD")
cio.track(5, "purchase", {"type": "socks"}, id="01HB4HBDKTFWYZCK01DMRSWRFD", timestamp=1561231234)

Mirrors the approach taken in customerio/customerio-ruby#136. Refs #98.

Test plan

• All 44 tests pass (7 new)
• id present in request body when provided, absent when not
• id absent when not provided
• timestamp set as top-level field when provided
• Invalid timestamps silently omitted
• track() with no data sends empty data dict
• track_anonymous() with id and timestamp

---

Note

Medium Risk
This is a breaking API change to the public track()/track_anonymous() signatures that will raise TypeError for existing keyword-attribute callers. It also changes event payload shape by introducing optional top-level id and timestamp, which could affect downstream deduplication/timing behavior if misused.

Overview
Adds support for optional top-level event id (for deduplication) and timestamp on CustomerIO.track() and CustomerIO.track_anonymous().

Breaking change: both methods now require custom event attributes to be passed via a data dict (instead of arbitrary **kwargs), defaulting to {} when omitted; invalid timestamp values are silently dropped.

Updates the README/CHANGELOG and expands tests to enforce the new call signatures and validate request bodies for data, id, and timestamp.

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