Support for UserInfo endpoint

Given an access_token, the UserInfo endpoint, as defined in https://openid.net/specs/openid-connect-core-1_0.html#UserInfo, may provide additional claims.

Some OIDC client libraries have built-in support for fetching UserInfo, e.g:

• oidc-client-ts: https://authts.github.io/oidc-client-ts/interfaces/OidcClientSettings.html#loaduserinfo
• spring-security-oauth2-client: https://docs.spring.io/spring-security/reference/servlet/oauth2/login/core.html#oauth2login-sample-boot-application

This could be a nice feature for leptos-oidc.

For example Entra ID does not provide the picture claim in the id_token but provides it through the UserInfo endpoint. I guess there are other use cases.