[DPE-5318] update user management

[MiaAltieri]

Issue:

1. Users created by clients get automatically deleted by MongoDB charm
2. Users for mongos K8s charm get automatically deleted by MongoDB charm

Solution:

Keep track of which users the charm should manage and only remove/update those users by bringing over k8s lib

Testing

Lib tested for all charms that use it i.e. MongoDB VM+K8s and Mongos K8s