feat(dart): update the userInfo api response type

[simeng-li]

Summary

update the userInfo API response type

Flutter SDK's fetchUserInfo endpoint response type is outdated. Update the response type. Add the fetchUserInfo method to the LogtoClient

Testing

UT case updated test locally

Checklist

• [ ] .changset
• [x] unit tests
• [ ] integration tests
• [x] docs WIP

OR

• [ ] This PR is not applicable for the checklist