Allow extending of authorization endpoint parameters.

[jebrosen]

(Split off from #11 (comment))

Some providers require or support additional parameters to be specified in the redirect request. One example is Slack, which has a second user_scope= parameter to specify scopes for the impersonated user in addition to the standard scope= which applies to the bot user.

This should be possible to do today by retrieving OAuth2<C> from managed state, calling OAuth2<C>::get_redirect(), and then manually mangling the Redirect URI. This is an unsavory option.

Perhaps it should be easier to add custom request parameters; something similar was done for TokenResponse (see #9 (comment)) to allow access to the exact data returned by the server. One option might be OAuth2<C>::get_redirect_custom(&self, cookies: &mut Cookies<'_>, scopes: &[&str], params: Params). Params would likely be an iterator of key-value pairs.

I would appreciate any further examples of providers that would benefit or require this functionality.

EDIT(2020-08-20): Removed mention of the automatically-generated login route, which was removed in 0.3.