Setup check for IMAP connection speed of provisioned accounts

[ChristophWurst]

Is your feature request related to a problem? Please describe.

As a nextcloud Mail admin I want to get warned when provisioned accounts may be slow.

Describe the solution you'd like

Add a setup check for this app that takes three random accounts per provisioning config, performs a simple reading operation on IMAP and measures the time of that. If the time is above x (to be determined), show a warning.

Describe alternatives you've considered

N/a

Additional context

We want to make it clearer when performance problems in Nextcloud are not caused by Nextcloud itself but infrastructure or external endpoints.