Rework oidc provider construction

[Jeidnx]

Rework the initialization code for oidc providers:

• Refactor the code to be more maintainable
• split github into its own init function since its a very special case
• Remove default values for {authorization,introspection,token} urls, since there is no standard default that could be used. All providers i know of support the discovery standard, which should be preferred instead of configuring these manually.
• Adds automatic construction of the callback url.
• Fixes oAuth can't parse client's sso urls #276

[Jeidnx]

Sorry i may have been a bit harsh with the words. I was still in my Software Engineering mindset and honestly a little frustrated since it took me some time to get this software to compile properly. (I will probably open an issue or a PR for the problems i encountered)

I refactored a lot of the configuration code, in my opinion this should be way better to read and maintain.
Github seems to be a very special case, so it only made sense that it gets it own function. The other branded providers comply with the discovery spec.
The callback_url really shouldn't be configurable, i fixed that and indicated it in the docs.

If you have any questions about the code please don't hesitate to ask

[Jeidnx]

Not sure why the CI fails on unchanged code. Runs fine on main: https://github.com/matrix-construct/tuwunel/actions/runs/21559255457/job/62120577532

[x86pup]

I think this is some new issue with the typos tool we use for detecting typos, not the first time something like this has happened. We will handle it ourselves don't worry. Thank you for the contribution.

[aazf]

@Jeidnx If possible, pls add username claim from OAuth provider.
some of provider return the username, not the preferred_username.

[jevolk]

When testing GitHub I 404'ed after the initial redirect. At a glance it might be that the base_path is not being applied. I can have a look but I have to merge the other pending items to main branch first.