initial fusionauth add

[alex-fusionauth]

Adds a provider for FusionAuth

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[thomasballinger]

Hi @alex-fusionauth! Thanks for contributing this, but we've found doing all of this as separate entrypoints in one package with optional peer dependencies isn't sustainable: the worry is that peer dependencies will end up conflicting and drag down everyone; so this should be a separate package.

Let me know what you'd like to do there: linking out to a walkthrough on your site seems like the easiest. The step we've had the most friction with is maintaining the setup flows for Clerk and Auth0. If you handled that part we can have steps in our docs that describe how to use FusionAuth. We'd prefer you take on the maintenance of this (and we promise to do our best not to break it if you're using public APIs) because developers get confused when onboarding flows change, but I understand it being nice to have clear instructions in our docs for you.