Use "name" claim in OIDC SSO for display name

[RaphaelRoumezin]

When creating a user from SSO, the code uses the OIDC claim "preferred_username" for the username, which is usually more of a login username than a display name, which is more how vaultwarden uses it. Entra ID, for instance, uses UPN, which might just be the email address.

vaultwarden/src/sso.rs

Line 291 in d6a3d53

let user_name = id_claims.preferred_username().or(user_info.preferred_username()).map(|un| un.to_string());

The OIDC spec indicates the field "name" as more fitting for a display name:
https://openid.net/specs/openid-connect-core-1_0.html#Claims

So we could either:

• Use the "name" claim in the code, with "preferred_username" as a fallback
• Make a config field to set the claim to be used for username definition