Allow passing headers, follow_redirects from ElevenLabs, AsyncElevenLabs to parent/base class

[pchng]

Summary

• The classes ElevenLabs and AsyncElevenLabs extend base classes that support the headers and follow_redirects as constructor parameters, but don't expose these on their own constructors, making it impossible to actually use these.
• Expose these in the child classes and pass onto parent constructors.
• Change should be backwards compatible because parameters are added as optional with default values identical to their parent classes.

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Note

Low Risk
Optional constructor kwargs with parent-aligned defaults; behavior change only for callers who previously could not pass these options.

Overview
ElevenLabs and AsyncElevenLabs now accept optional headers and follow_redirects and pass them to their base classes, fixing TypeError when callers used kwargs the base already supported but the public subclasses did not forward.

Constructor docs are updated for both sync and async clients. Defaults match the parent (headers=None, follow_redirects=True), so existing call sites stay unchanged.

New tests in test_client_headers.py cover forwarding to the client wrapper, merged request headers, httpx redirect behavior, and keyword-only signatures for both client types.

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