Add additional option for kiro-cli apikey usage (multi Tennant support)

[marcelloceschia]

Adds support for Kiro API keys (ksk_*) as a new authentication method. The gateway can now run in fully stateless mode — no server-side credentials required. Users pass their own API key as a Bearer token, and it's forwarded directly to Kiro API with the tokentype: API_KEY header.

Motivation

Enable a single shared gateway instance where multiple users connect with their own Kiro subscription via API keys generated from kiro-cli settings.

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[marcelloceschia]

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[marcelloceschia]

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Thanks for the PR! 🎉

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Thanks for the PR! 🎉

Before merge, we need a one-time CLA confirmation.
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