[enhancement]: Allow changing default http path

[pidario]

Which feature or improvement would you like to request?

I'd like to be able to configure stalwart so that its admin web panel, API and JMAP endpoints are reachable not at the root path.

For instance:
instead of: https://mail.example.com/
I would like to open the admin panel at https://example.com/mail/.
This simplifies DNS configuration and certificates.

Opening this issue as suggested in #976 by @mdecimus

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