Add stats UTM endpoint

[adalpari]

Description

Add a new WP.com REST API endpoint for UTM stats at /sites/{site_id}/stats/utm/{utm_keys}. This follows the existing stats endpoint patterns in the codebase.

Changes

• Add StatsUtmKey enum supporting utm_source, utm_medium, and utm_campaign
• Add StatsUtmKeys newtype for comma-separated UTM keys used as a URL path segment
• Add StatsUtmParams with support for max, date, days, start_date, post_id, and query_top_posts query parameters
• Add StatsUtmResponse, StatsUtmPost response types with top_utm_values and top_posts fields
• Handle empty array/object responses via deserialize_empty_array_or_hashmap
• Register the endpoint in client.rs, endpoint.rs, and mod.rs

Test plan

• Unit tests for UTM key serialization (single, multiple, all keys)
• Unit tests for query parameter serialization (full, minimal, query_top_posts=false)
• Deserialization tests for single-key, multiple-keys, triple-keys, empty, and empty-arrays JSON fixtures

🤖 Generated with Claude Code

[wpmobilebot]

XCFramework Build

This PR's XCFramework is available for testing. Add to your Package.swift:

.package(url: "https://github.com/automattic/wordpress-rs", branch: "pr-build/1253")

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