Standard Header Key for Extension-based Endpoint Selection

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This project utilizes an endpoint selection mechanism for gateway extensions. Other extensions will likely follow with similar behavior, we should create a standardized, expected header key that indicates the intent of selection.

This issue is intended to centralize discussion around this topic, with the lead choice being: x-gateway-destination-endpoint

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CCs: @robscott @arkodg @danehans @shaneutt

Feel free to add others that may be relevant