[lib] utility function to get array of OneTimeKeys from an OLMOneTimeKeys object

Summary:
we need to do this in web/native(/keyserver?) so created a small utility function

Depends on D9127

Test Plan: called the function and got back an array of keys

Reviewers: marcin, ashoat

Reviewed By: marcin, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D9129