Syncing certificates between nodes in a cluster

[forfitoo]

Is your feature request related to a problem? Please describe.
Currently, when using nginx-ui cluster feature there is no option to synchronize certificates between nodes in a cluster. For example, whenever we are renewing our "letsencrypt" certificates on a master node, they are not getting synced back to our backup nodes.

This raises two problems:

1. If we sync those certificates using external tools like rsync - the certificates are not showing up on "Certificates list" page on nginx-ui web interface.
2. After we imported them on our backup node - in case this backup node temporarily becomes the master and the certificates will need renewal it won't be able to do so because the dates on the certificates will not be present in certificates database of nginx-ui.

Describe the solution you'd like
Similarly to config synchronization, there should be a feature that allows you to manually/automatically sync certificates between the nodes.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
Add any other context or screenshots about the feature request here.