[native] Generate Keys - add Persist structure
[native] Generate Keys - add Persist structure

Summary:
Adding persist structure to the comm repo. This is used to pickling an account and its sessions.
The original code can be found [here](https://github.com/karol-bisztyga/olm_tests/tree/main/karol_tests)

Test Plan: none

Reviewers: palys-swm, ashoat

Reviewed By: palys-swm, ashoat

Subscribers: ashoat, palys-swm, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D1842

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