feature request: Make preferred_username standard claim have the same value as username by default

[TheLonelinessOfHS]

What problem did you meet?

Some APPs rely on the standard preferred_username claim to work. An example is Mealie. Currently, Logto only returns a username claim by default when preferred_username is not set.

Describe what you'd like Logto to have

I would like logto to return a standard preferred_username that has the same value as the username by default.

[github-actions]

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