feat(ads-client): add data_dir support with context_id persistence

[Almaju]

Introduce AdsClientBuilder with a data_dir option that allows the component to manage its own persistent files (cache DB, context_id) in a dedicated ads-client/ subdirectory.

Data dir resolution follows a fallback chain:

1. Explicit data_dir provided via builder
2. Parent directory of legacy cache_config.db_path
3. $HOME environment variable (iOS, macOS, Linux, Windows)
4. $APPDATA / $LOCALAPPDATA (Windows fallback)

When resolved, context_id is persisted to a file so the 3-day rotation actually works across app restarts (previously a fresh UUID was generated on every client init, making rotation meaningless).

Changes:

• New client/builder.rs: AdsClientBuilder with all resolution, cache building, and context_id persistence logic
• Simplified AdsClient::new() to accept pre-built components
• Added try_build() to MozAdsClientBuilder (returns Result)
• Deprecated build() in favor of try_build()
• Added data_dir() method to MozAdsClientBuilder
• Removed unused AdsCacheConfig and AdsClientConfig types
• Added BuildError enum (NoDataDir, CreateDataDir)

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