[client] Add per-request extra headers

[juharris]

Same as alexrudall#633

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Allow passing custom HTTP headers on individual API calls rather than only at client initialization time. This enables varying headers per request while reusing a single cached client instance.

Changes:

• HTTP#get, #post, #json_post, #multipart_post, #delete accept extra_headers: {}
• Client#chat, #embeddings, #completions, #moderations accept extra_headers: {}
• Added tests for extra_headers functionality

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• Have you checked to ensure there aren't other open Pull Requests for the same update/change?
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