feat(ads-client): ignore context_id in HTTP cache key generation

[Almaju]

Remove cycle_context_id() API and modify cache to ignore context_id field in request bodies when generating cache keys. This prevents unnecessary cache invalidation when context_id rotates automatically.

• Remove cycle_context_id() public API (breaking change)
• Strip context_id from request body before hashing for cache key
• Add tests for context_id-agnostic caching behavior
• Update documentation to remove cycle_context_id() method

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[Almaju]

@luc-lisi An idea for a not so complicated refactor instead of the quick & dirty fix:
HttpCache could become HttpCache<T> where T: Into<Request> & T: Into<RequestHash>
This way for our use-case we could have HttpCache<AdRequest> and then implement the hash trait on AdRequest directly so that we don't pollute HttpCache with our specific requirement of ignoring context-id.