Support easy auth

[stevetemple]

Describe your changes

Add support for easy auth

Link to issue

#57

Checklist before requesting a review

• I have performed a self-review of my code

[apa-agridea]

I tested this branch with Umbraco 18.0.2 behind Azure App Service Easy Auth. The base Easy Auth flow works, but I needed two small compatibility fixes to complete the external-login flow:

• Set AuthenticationProperties.Items["LoginProvider"] = Scheme.Name when creating the Easy Auth authentication ticket. ASP.NET Identity uses this to reconstruct ExternalLoginInfo after the callback.

• Normalize the raw Entra email JWT claim to ClaimTypes.Email when that standard claim is absent, with preferred_username as a fallback.
  The handler currently creates its identity directly from raw ID-token claims. Umbraco auto-linking looks for ClaimTypes.Email, so it otherwise returns FailedNoEmail and the backoffice begins another challenge.

Our Entra ID token included email and the configured group IDs; after these two changes, the user was auto-linked and signed in successfully.