[keyserver] check if there are any unpublished prekeys and publish them during rotation

Summary:
during prekey rotation on keyserver, check if we have an unpublished prekey, and if we do, upload and mark them as published, then return

this matches the rotation logic on native

Depends on D12441

Resolves https://linear.app/comm/issue/ENG-6490/resolve-discrepancy-between-prekey-rotation-logics-on-native-and-the

Test Plan: called generate_prekey() to generate new content and notif prekeys. confirmed that the if condition was satisfied and the newly generated prekeys were uploaded to identity and marked as published

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12442