feat: Implement `oauthenticator.OAuthenticator.refresh_user` method

[Wykiki]

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This MR aims to implement the refresh_user()'s Authenticator method in OAuthenticator class.

Following #398 comment.

At the time of writing, nothing has been tested, and provider specific code would be needed, as the OAuth2 spec does not force the expires_in field in the access_token response body.

I may not have bandwidth soon to work again on this, feel free to take the lead !

[yuvipanda]

I'll be super excited to try get this to eventually land :)

[Wykiki]

@yuvipanda I don't have bandwidth to work on it anymore, so feel free to take over the code !

[yuvipanda]

@Wykiki I'll poke around and try to find someone!

[yuvipanda]

And thank you for your contribution :)

[epstein6]

Hello! Was any progress made here? This would be useful for my work, and this looks like a fairly complete implementation (thanks!).

[epstein6]

I've been using this with gitlab with a couple of compatibility changes (adding the redirect_url and the client info) and it seems to work. Are there any other particular changes that would need to be done here for this to be merged?