feat(auth): Adds Twitch identity provider support

[89pleasure]

This PR adds Twitch OAuth authentication support to the serverpod_auth module. It implements a complete Twitch identity provider (IDP) with PKCE (Proof Key for Code Exchange) flow for enhanced security across all platforms. This is possible to the great work of @vfiruz97

Key features:

Cross-platform support (iOS, Android, Web, macOS, Windows, Linux)
Customizable sign-in button widget with Twitch branding
Complete server-side token exchange, user authentication, and account management
Integration with existing Serverpod auth system
Comprehensive test coverage including integration tests

No new dependencies added

Pre-launch Checklist

• I read the Contribute page and followed the process outlined there for submitting PRs.
• This update contains only one single feature or bug fix and nothing else. (If you are submitting multiple fixes, please make multiple PRs.)
• I read and followed the Dart Style Guide and formatted the code with dart format.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///), and made sure that the documentation follows the same style as other Serverpod documentation. I checked spelling and grammar.
• I added new tests to check the change I am making.
• All existing and new tests are passing.
• Any breaking changes are documented below.

If you need help, consider asking for advice on the discussion board.

Breaking changes

This is a new feature. I changed the exchangeCodeForToken to a generic method to support a different response schemas. Twitch is using a response token approach similar to Apple which won't work the way it is implemented right now. Since im new to dart development this needs further testing.

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[89pleasure]

I have read the CLA Document and I hereby sign the CLA

[89pleasure]

Still in progress, just wanted to save and push my work already :)

[marcelomendoncasoares]

Thanks for this valuable contribution, @89pleasure! Since you had to base your work from #4546, the diff is carrying a lot of changes and will make the revision harder. We are still waiting for the docs, but I'll merge #4546 already so you can rebase from main!

[89pleasure]

Thanks for this valuable contribution, @89pleasure! Since you had to base your work from #4546, the diff is carrying a lot of changes and will make the revision harder. We are still waiting for the docs, but I'll merge #4546 already so you can rebase from main!

Thanks @marcelomendoncasoares ! Im on it. But I recognized that the merge into main has changes the PR does not have. Seems to be force pushed too. That makes a rebase on main pretty hard to distinguish the changes now.

[marcelomendoncasoares]

Thanks @marcelomendoncasoares ! Im on it. But I recognized that the merge into main has changes the PR does not have. Seems to be force pushed too. That makes a rebase on main pretty hard to distinguish the changes now.

You should do a rebase using the --onto option to only move your commits on top of latest main. Since all merges in Serverpod are done with squash, trying to rebase the actual branch will carry all commits of the last branch as well - down until the common ancestor of the main commit where the GitHub branch was based upon. Also, if your branch was not based on the last commit of the other PR, then you will indeed face conflicts on newer code.

[vfiruz97]

We are still waiting for the docs, but I'll merge #4546 already so you can rebase from main!

Thanks @marcelomendoncasoares, I am happy, finally to get my PR merged! The docs on its way. Needs your attention :) serverpod/serverpod_docs#402

[89pleasure]

Thanks @marcelomendoncasoares ! Im on it. But I recognized that the merge into main has changes the PR does not have. Seems to be force pushed too. That makes a rebase on main pretty hard to distinguish the changes now.

You should do a rebase using the --onto option to only move your commits on top of latest main. Since all merges in Serverpod are done with squash, trying to rebase the actual branch will carry all commits of the last branch as well - down until the common ancestor of the main commit where the GitHub branch was based upon. Also, if your branch was not based on the last commit of the other PR, then you will indeed face conflicts on newer code.

I look into it, I already did a rebase, but for some reason the commits of the GitHub idp are still in here. I'll redo it with the --onto flag. @marcelomendoncasoares

[marcelomendoncasoares]

@89pleasure Now that we have merged the GitHub IDP with the changes you proposed, my suggestion will be for you to rebase your commits on latest main and open a new PR with a clean history. This will simplify the conflict resolution for you and also facilitate the revision a lot. What do you think?

[89pleasure]

@marcelomendoncasoares Yes! That's great news. I was waiting for this. Also I'm currently very busy at work so I will take care of that on the weekend.