feature request: Make preferred_username a standard claim, with the username as it's default

[chark1es]

What problem did you meet?

Using selfhosted applications, such as gitea / forgejo, they use preferred_username as the standard.

Describe what you'd like Logto to have

I want Logto to be able to set preferred_username as the user's username by default. If they want to update it, they can always use the Management API, but since many selfhosted apps use preferred_username, it would make sense to include it

This functionality should be included in the OSS version too

This is related to the issue #6410.