Support for user registration - prompt=create

[jhartmann123]

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Describe the solution you'd like

OpenIddictConstants.Prompts currently does not support the value create used for initiating user registration.

Spec: https://openid.net/specs/openid-connect-prompt-create-1_0.html

As it looks like on a first glance, the Prompts-class is just missing the value, as the rest is basically a normal authorization / login flow.

Additional context

No response

[kevinchalet]

Hey,

Enhancement suggestions are reserved to sponsors but I liked the idea and decided to implement the complete specification in OpenIddict 6.0: #2197.

[jhartmann123]

Oh, sorry, I was sure my company sponsored you the last ~1.5 years, but looking at the GitHub sponsors it doesn't look like that. I'll look into that.

Thanks for implementing that feature anyway! 💯