feat(client): support extra headers from model resolution#1575

RoseSecurity wants to merge 3 commits into

CopilotC-Nvim:mainCopilotC-Nvim/CopilotChat.nvim:mainfrom

RoseSecurity:use-auto-model-lookup-by-defaultRoseSecurity/CopilotChat.nvim:use-auto-model-lookup-by-defaultCopy head branch name to clipboard