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SurveyJS
Open-Source JSON Form Builder to Create Dynamic Forms Right in Your App. With SurveyJS form UI libraries, you can build and style forms in a fully-integrated drag & drop form builder, render them in your JS app, and store form submission data in any backend, inc. PHP, ASP.NET Core, and Node.js.
Passport is an Express middleware for authentication. It supports pluggable "strategies" for supporting different types of authentication, from basic username/password to OAuth and many others.
https://www.npmjs.com/package/passport
Looks like C++ client support is still a request? https://github.com/supabase/supabase/discussions/5
The ORY SDKs are auto-generated using OpenAPI, so you could probably request one or generate one yourself (https://github.com/ory/sdk). I just put in a request for a Rust SDK and was able to auto-generate one locally for testing purposes pretty easily.
Not open source, but free as in beer: https://curity.io/product/community/
It's a complex server, but if you need to connect to lots of third party systems it may be a good choice as it has support for all sorts of things and its plugins are open-source (e.g. BitBucket plugin: https://github.com/curityio/bitbucket-authenticator)