refactor(ads-client): housekeeping - sort fields/methods, add UniFFI defaults, split docs, add Client in HttpCache

[Almaju]

• Sort struct fields alphabetically (HttpCache)
• Sort methods: constructors → pub (alpha) → private (alpha) in HttpCache, AdsClient, MARSClient
• Add #[uniffi(default = None)] to optional FFI fields for better consumer ergonomics (iab_content, cache config fields, ttl_seconds)
• Replace manual Default impl with #[derive(Default)] (clippy)
• Split usage.md into per-language files (JavaScript, Kotlin, Swift) with only consumer-facing examples; update README links
• Add viaduct::Client in HttpCache to make it compatible with future OHTTP impl

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