add advancement API for offlineplayer

[ghost]

Is your feature request related to a problem?

can't access advancements of an offline player easier

Describe the solution you'd like.

add methods, same as player.getstatistic

Describe alternatives you've considered.

loading json blah blah hassle

Agreements

I have searched for and ensured there isn't already an open issue regarding this.
I have ensured the feature I'm requesting isn't already in the latest supported Paper build.

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.