fix: use Duration type for PersistentKeepalive field in InterfaceWireguardPeer

[maksym-nazarenko]

Fixes #221

In MikroTik documentation, the persistent_keepalive is defined as Integer: A seconds interval, between 1 and 65535 inclusive.

In fact, after client sets it, in all subsequent reads this field becomes a duration:

Set value	Read value
20	20s
123	2m3s
3602	1h2s

This PR changes PersistentKeepalive field type to our internal Duration type that can handle such normalizations.