gptel-gh: use the host dynamically

[kiennq]

This allows to use the correct host url that's set in the token, ex: api.enterprise.githubcopilot.com

[karthink]

@kiennq thanks for the PR. Does the host url in the token change all the time, or is it otherwise unpredictable? I'm asking since you're repeatedly setting (gptel-backend-host gptel-backend) (and the protocol) with every request in this PR.

[kiennq]

It's depending on the git hub chat token, which will be refreshed time to time. I think it's not going to change but is there issue with always setting those values?

[karthink]

It's depending on the git hub chat token, which will be refreshed time to time.

I'm surprised the host can change between token refreshes. Is this really the case? I would have assumed it would be enough to set it once from the token.

I think it's not going to change but is there issue with always setting those values?

The header function is setting the backend host and protocol by side effect -- it just seems confusing and the wrong place to do it.